How do we know the number of shorted shares of a stock?
For a company listed on NASDAQ, the numbers are published on NASDAQ's site. The most recent settlement date was 4/30/2013, and you can see that it lists 27.5 million shares as held short.  NASDAQ gets these numbers from FINRA member firms, which are required to submit them to the exchange twice a month: Each FINRA member firm is required to report its “total” short interest positions in all customer and proprietary accounts in NASDAQ-listed securities twice a month. These reports are used to calculate short interest in NASDAQ stocks. FINRA member firms are required to report their short positions as of settlement on (1) the 15th of each month, or the preceding business day if the 15th is not a business day, and (2) as of settlement on the last business day of the month.* The reports must be filed by the second business day after the reporting settlement date. FINRA compiles the short interest data and provides it for publication on the 8th business day after the reporting settlement date.
"The reason for selling a stock ""short"", is for when you believe the stock value will decrease in the near future. Here is an example: Today Exxon-Mobile stock is selling for $100 / share. You are expecting the price to decrease, so you want to short the stock, which means your broker (i.e. eTrade, etc) allows you to borrow shares without paying money, and those shares are transferred into your account, and then you sell them and receive money for the sale. But you didn't actually own those shares, you only borrowed them, so you need to return the shares to your broker sometime in the future.  Let's say you borrow 10 shares @ $100, and you sell them at the market price of $100, you receive $1,000 in your account. But you owe your broker 10 shares, which you need to return sometime in the future. A few days later, the share price has decreased to $80. Now you can buy 10 shares from the market at a total cost of $800. You get 10 shares, and return those shares to your broker. Since you originally took in $1,000, and you just paid out $800, you keep a resulting profit of $200" "As the other answer said, the person who owns the lent stock does not benefit directly.  They may benefit indirectly in that brokers can use the short lending profits to reduce their fees or in that they have the option to short other stocks at the same terms.   Follow-up question: what prevents the broker lending the shares for a very short time (less than a day), pocketing the interest and returning the lenders their shares without much change in share price (because borrowing period was very short). What prevents them from doing that many times a day ? Lack of market.   Short selling for short periods of time isn't so common as to allow for ""many"" times a day.  Some day traders may do it occasionally, but I don't know that it would be a reliable business model to supply them.  If there are enough people interested in shorting the stock, they will probably want to hold onto it long enough for the anticipated movement to happen.   There are transaction costs here.  Both fees for trading at all and the extra charges for short sale borrowing and interest.  Most stocks do not move down by large enough amounts ""many"" times a day.  Their fluctuations are smaller.  If the stock doesn't move enough to cover the transaction fees, then that seller lost money overall.  Over time, sellers like that will stop trading, as they will lose all their money.   All that said, there are no legal blocks to loaning the stock out many times, just practical ones.  If a stock was varying wildly for some bizarre reason, it could happen." There is no difference.  When dealing with short positions, talking about percentages become very tricky since they no longer add up to 100%.  What does the 50% in your example mean?  Unless there's some base amount (like total amount of the portfolio, then the percentages are meaningless.  What matters when dealing with long and short positions is the net total - meaning if you are long 100 shares on one stock trade and short 50 shares on another, then you are net long 50 shares. "There are two primary reasons shares are sold short:  (1) to speculate that a stock's price will decline and (2) to hedge some other related financial exposure. The first is acknowledged by the question.  The second reason may be done for taxes (shorting ""against the box"" was once permitted for tax purposes), for arbitrage positions such as merger arbitrage and situations when an outright sale of stock is not permitted, such as owning restricted stock such as employer-granted shares. Why would a shareholder lend the investor the shares? The investor loaning his stock out to short-sellers earns interest on those shares that the borrower pays.  It is not unusual for the annualized cost of borrowing stock to be double digits when there is high demand for heavily shorted shares.  This benefit is however not available to all investors." When you buy a stock, the worst case scenario is that it drops to 0.  Therefore, the most you can lose when buying a stock is 100% of your investment. When you short a stock, however, there's no limit on how high the stock can go.  If you short a stock at 10, and it goes up to 30, then you've lost 200% on your investment.  Therefore shorting stocks is riskier than buying stocks, since you can lose more than 100% of your investment when shorting. because the price might go up, but it will never be as big of a change as a regular price drop i suppose... That is not true.  Stocks can sometimes go up significantly (50-100% or more) in a very short amount of time on a positive news release (such as an earnings or a buyout announcement).  A famous example occurred in 2008, when Volkswagen stock quintupled (went up 400%) in less than 2 days on some corporate news: Porsche, for some reason, wants to control Volkswagen, and by building up its stake has driven up the price. Hedge funds, figuring the share price would fall as soon as Porsche got control and stopped buying, sold a lot of VW shares short. Then last weekend, Porsche disclosed that it owned 42.6 percent of the stock and had acquired options for another 31.5 percent. It said it wanted to go to 75 percent. The result: instant short-squeeze. The German state of Lower Saxony owns a 20 percent stake in VW, which it said it would not sell. That left precious few shares available for anyone else. The shorts scrambled to cover, and the price leaped from about €200, or about $265, to above €1,000. In order to short a stock, you have to borrow the number of shares that you're shorting from someone else who holds the shares, so that you can deliver the shares you're shorting if it becomes necessary to do so (usually; there's also naked short selling, where you don't have to do this, but it's banned in a number of jurisdictions including the US). If a stock has poor liquidity, or is in high demand for shorting, then it may well be impossible to find anyone from whom it can be borrowed, which is what has happened in this instance. In less than two decades, more than half of all publicly traded companies have disappeared. There were 7,355 U.S. stocks in November 1997, according to the Center for Research in Security Prices at the University of Chicago’s Booth School of Business. Nowadays, there are fewer than 3,600.  A close look at the data helps explain why stock pickers have been underperforming. And the shrinking number of companies should make all investors more skeptical about the market-beating claims of recently trendy strategies.  Back in November 1997, there were more than 2,500 small stocks and nearly 4,000 tiny “microcap” stocks, according to CRSP. At the end of 2016, fewer than 1,200 small and just under 1,900 microcap stocks were left.  Most of those companies melted away between 2000 and 2012, but the numbers so far show no signs of recovering.  Several factors explain the shrinking number of stocks, analysts say, including the regulatory red tape that discourages smaller companies from going and staying public; the flood of venture-capital funding that enables young companies to stay private longer; and the rise of private-equity funds, whose buyouts take shares off the public market.  For stock pickers, differentiating among the remaining choices is “an even harder game” than it was when the market consisted of twice as many companies, says Michael Mauboussin, an investment strategist at Credit Suisse in New York who wrote a report this spring titled “The Incredible Shrinking Universe of Stocks.”  That’s because the surviving companies tend to be “fewer, bigger, older, more profitable and easier to analyze,” he says — making stock picking much more competitive.  Consider small-stock funds. Often, they compare themselves to the Russell 2000, an index of the U.S. stocks ranked 1,001 through 3,000 by total market value. “Twenty years ago, there were over 4,000 stocks smaller” than the inclusion cutoff for the Russell 2000, says Lubos Pastor, a finance professor at the University of Chicago. “That number is down to less than 1,000 today.”  So fund managers have far fewer stocks to choose from if they venture outside the index — the very area where the best bargains might be found. More money chasing fewer stocks could lead some fund managers to buy indiscriminately, regardless of value.  Eric Cinnamond is a veteran portfolio manager with a solid record of investing in small stocks. Last year, he took the drastic step of shutting down his roughly $400 million mutual fund, Aston/River Road Independent Value, and giving his investors their money back.  “Prices got so crazy in small caps, I fired myself,” he says. “My portfolio was 90% in cash at the end, because I couldn’t find anything to buy. If I’d kept investing, I was sure I’d lose people their money.”  He adds, “It was the hardest thing I’ve ever done professionally, but I didn’t feel I had a choice. I knew my companies were overvalued.”  Mr. Cinnamond hopes to return to the market when, in his view, values become attractive again. He doesn’t expect recent conditions to be permanent.  The evaporation of thousands of companies may have one enduring result, however — and it could catch many investors by surprise.  Most research on historical returns, points out Mr. Mauboussin, is based on the days when the stock market had twice as many companies as it does today. “Was the population of companies so different then,” he asks, “that the inferences we draw from it might no longer be valid?”  So-called factor investing, also known as systematic or smart-beta investing, picks hundreds or thousands of stocks at a time based on common sources of risk and return. Among them: how big companies are, how much their shares fluctuate, how expensive their shares are relative to asset value and so on.  But the historical outperformance of many such factors may have been driven largely by the tiniest companies — exactly those that have disappeared from the market in droves.  Before concluding that small stocks or cheap “value” stocks will outrace the market as impressively as they did in the past, you should pause to consider how they will perform without the tailwinds from thousands of tiny stocks that no longer exist.  The stock market has more than tripled in the past eight years, so the eclipse of so many companies hasn’t been a catastrophe. But it does imply that investing in some of the market’s trendiest strategies might be less profitable in the future than they looked in the past. The worth of a share of stocks may be defined as the present cash value of all future dividends and liquidations associated therewith.  Without a crystal ball, such worth may generally only be determined retrospectively, but even though it's generally not possible to know the precise worth of a stock in time for such information to be useful, it has a level of worth which is absolute and not--unlikely market price--is generally unaffected by people buying and selling the stock (except insofar as activities in company stock affect a company's ability to do business). If a particular share of stock is worth $10 by the above measure, but Joe sells it to Larry for $8, that means Joe gives Larry $2.  If Larry sells it to Fred $12, Fred gives Larry $2.  The only way Fred can come out ahead is if he finds someone else to give him $2 or more.  If Fred can sell it to Adam for $13, then Adam will give Fred $3, leaving Fred $1 better off than he would be if he hadn't bought the stock, but Adam will be $3 worse off. The key point is that if you sell something for less than it's worth, or buy something for more that it's worth, you give money away.  You might be able to convince other people to give you money in the same way you gave someone else money, but fundamentally the money has been given away, and it's not coming back. It this a real situation or is it a made up example? Because for a stock that has a last traded priced of $5 or $6 and volume traded over $4M (i.e. it seems to be quite liquid), it is hardly likely that the difference from bid to ask would be as large as $1 (maybe for a stock that has volume of 4 to 5 thousand, but not for one having volume of 4 to 5 million). In regards to your question, if you were short selling the order would go in exactly the same as if you were selling a stock you owned. So your order would be on the ask side and would need to be matched up with a price on the bid side for there to be a trade. Brokerage firms are required to report the number of shares being shorted.  This information is reported to the exchange (NYSE of NASDAQ) and is made public.  Most financial sites indicate the number of shares being shorted for a particular stock.  The image below from Yahoo finance shows 3.29 million shares of CMG were being shorted at the close of 9-28-2012.  This is over 12% of the total outstanding shares of CMG.  For naked short selling additional information is tracked.  If the brokerage is unable to borrow shares to deliver before the settlement date of a short sale then the transaction is recorded as fails-to-deliver.  No money or shares are exchanged since the brokerage is unable to deliver the shares that were agreed upon.  A large amount of fails-to-deliver transactions for a stock usually indicates an excessive amount of naked shorting.  When investors and brokerage firms start to aggressively short a stock they will do so without having borrowed the shares to sell.  This will result in a large amount of naked short selling.  When there are a large number of naked short sellers not all the sellers will be able to borrow the necessary shares before the settlement date and many fails-to-deliver transactions will be recorded. The SEC records the number of fails-to-deliver transactions. The table below summarizes the fails-to-deliver transactions from 1-1-2012 through 9-14-2012 (data obtained from here). The “Ext Amount” column shows the total dollar value of the transactions that failed ( i.e. Fail Qty * Share price ).  The “Volume” column is the total number of shares traded in the same time period.  The “% Volume” shows the percentage of shares that failed to deliver as a percentage of the total market volume. The table orders the data in descending order by the quantity of shares that were not delivered.  Most of the companies at the top of the list no longer exist.  For many of these companies, the quantity of shares that failed to deliver where many multiples of the number of shares traded during the same time period.  This indicates massive naked short selling as many brokerages where unable to find shares to borrow before the settlement date. More information here. You can't. If there was a reliable way to identify an undervalued stock, then people would immediately buy it, its price would rise and it wouldn't be undervalued any more. "To summarize, there are three basic ways: (3) is the truly dangerous one. If there is a lot of short interest in a stock, but for some reason the stock goes up, suddenly a lot of people will be scrambling to buy that stock to cover their short position -- which will drive the price up even further, making the problem worse.  Pretty soon, a bunch of smart rich guys will be poor guys who are suddenly very aware that they aren't as smart as they thought they were.  Eight years ago, such a ""short squeeze"", as it's called, made the price of VW quadruple in two days.  You could hear the Heinies howl from Hamburg to Haldenwanger. There are ways to protect yourself, of course.  You can go short but also buy a call at a much higher price, thereby limiting your exposure, a strategy called a ""straddle"", but you also reduce your profit if you guessed right. It comes down to, as it always does, do you want to eat well, or to sleep well?" "You can't do this automatically; you want to understand whether the drop is from a short-term high. is likely to be a short-term low, or reflects an actual change in how folks expect the company to do in the future. Having said that, some people do favor a strategy which resembles this, betting on what are known as ""the dogs of the Dow"" in the assumption that they're well trusted but not as strongly sought and therefore perhaps not bid up as strongly. I have no opinion on it; I'm just mentioning it for comparison." James Roth provides a partial solution good for stock picking but let's speed up process a bit, already calculated historical standard deviations: Ibbotson, very good collection of research papers here, examples below Books Matt Levine talked about a cute scam that this resembles, a kind of extended short squeeze. You manipulate up the stock of a company, so that it's obviously way above the fundamental value. Word will get out. Then the shorts come in. But the value remains stubbornly high. All the stock is held by a few insiders, but they didn't manipulate the stock price to do a pump and dump. They did it to milk the shorts on borrow cost.