Why is the volume highest at the beginning and end of a trading day?
While volume per trade is higher at the open and to a lesser extent at the close, the overall volume is actually lower, on average. Bid ask spreads are widest at the open and to a lesser extent at the close. Generally, bid ask spreads are inversely proportional to overall volumes. Why this is the case hasn't been sufficiently clearly answered by academia yet, but some theories are that One of the fundamental of technical analysis suggests that holding a security overnight represents a huge commitment.  Therefore it would follow that traders would tend to close their positions prior to market close and open them when it opens.
"Trading at the start of a session is by far higher than at any other time of the day. This is mostly due to markets incorporating news into the prices of stocks. In other words, there are a lot of factors that can affect a stock, 24 hours a day, but the market trades for only 6.5 hours a day. So, a lot of news accumulates during the time when people cannot trade on that news. Then when markets finally open, people are able to finally trade on that news, and there is a lot of ""price discovery"" going on between market participants. In the last minutes of trading, volumes increase as well. This can often be attributed to certain kinds of traders closing out their position before the end of the day. For example, if you don't want to take the risk a large price movement at the start of the next day affecting you, you would need to completely close your position." This happens on dark pools quite often. If I am a large institutional investor with tens of millions of shares, I may want to unload slowly and limit the adverse affects on the price of the stock. Dark pools offer anonymity and have buyers / sellers that can handle large volume.  In the case of a day trader, they often trade stocks with light volume (since they have large fluctuations that can be quite profitable throughout the session). At the end of the session, many traders are unwilling to hold positions on margin and want to unload fast. Consider the price history to be the sum of short term movements and long term movements. If you hold a stock for a long time you will benefit (or lose) from its long term movement.  If a sufficiently large and very good short term trader existed he would tend to reduce short term volatility, eventually to nearly zero. At that point, the price would rise gently over the course of the day in line with the long term variation in price. Presumably robot traders will increase the time horizon of their trades when they have exhausted the gains they can make from short term trades. This is to protect your position in specific highly volatile market conditions.  If the stock is free falling and you only have a stop order at $90, it's possible that this order could be filled at $50 or even less. The limit is to protect you from that, as there are certain very specific times where it's better to just hold the stock instead of taking a huge loss (ie when price is whipsawing). "Volume is measured in the number of shares traded in a given day, week, month, etc. This means that it's not necessarily a directly-comparable measure between stocks, as there's a large difference between 1 million shares traded of a $1 stock ($1 million total) and 1 million shares traded of a $1000 stock ($1 billion total). Volume as a number on its own is lacking in context; it often makes more sense to look at it as an overall dollar amount (as in the parentheses above) or as a fraction of the total number of shares in the marketplace. When you see a price quoted for a particular ticker symbol, whether online, or on TV, or elsewhere, that price is typically the price of the last trade that executed for that security. A good proxy for the current fair price of an asset is what someone else paid for it in the recent past (as long as it wasn't too long ago!). So, when you see a quote labeled ""15.5K @ $60.00"", that means that the last trade on that security, which the service is using to quote the security's price, was for 15500 shares at a price of $60 per share. Your guess is correct. The term ""institutional investor"" often is meant to include many types of institutions that would control large sums of money. This includes large banks, insurance companies, pooled retirement funds, hedge funds, and so on." It's a really big stretch to assume that a particular 10Q release will have the exact same impact on trading volume, trading price, and volatility as a prior release, in a different quarter, under a different set of circumstances and a different set of results, which is what you seem to be implying when you suggest that a lack of volume in response to one quarter's results should therefore be used as a benchmark.   I think traders and the HFT firms are a lot smarter than you think. Market price simply depends on your order side.  If you are placing a buy order the market price is the lowest ask, if you are placing a sell order the market price is the highest bid.  If your order is larger than the volume then you'd need to also consider the next lowest ask or next highest bid until you've fulfilled your order volume. This is often the case where traders are closing out short positions they don't want to hold overnight, for a variety of reasons that matter to them.  Most frequently, this is from day traders or high-frequency traders settling their accounts before the markets close. Trading at the start of the day is highest because of news flows that may have come after the close of the previous day.  And trading at the end of the day is highest because of expected news flows after closing hours. Moreover, there are many day traders who buy in the morning without making any payment for purchase and such traders have to sell by evening or else they will have to make the payment for the purchases which they have made. Besides overnight news events and auction mechanisms there is a more fundamental reason the price of a stock is always moving.  Theoretically the stock price will move slightly even in the unlikely scenario that absolutely nothing of interest happens during the entire night.  Let me go into that in some more detail: Stock valuation using Discounted Cash FLow One of the fundamental reasons that stock value is constantly changing is because underneath every stock there is a company that expects to make some kind of profit or loss in the future. We have to go into the fundamentals of stock value to understand why this is important: One popular way to determine the value of a stock is by looking into the future and summing up all the earnings (or cashflows) it has yet to produce. You have to reduce each amount by a certain factor that gets larger for payments that are farther into the future. Think of it this way: a dollar in hand now is better than a dollar that you get tomorrow. This method of valuation is called Discounted Cash Flow (abbr. DCF; see the wikipedia article on DCF) Time's effect on stock value Now take the Close price C, and the open price O. Let's assume that since there has been no news, the expectations for future earnings are the same for C and O. Remember that the discounting factor for these earnings is dependent on the time until the cashflow occurs? For O, this time is slightly shorter than for C, and therefore the value will be slightly higher (or lower, when the company is expected to incur losses). So now you can see that even without all the external forces that continuously push and pull on the stock price, a stock still changes in value over time. Hope this helps. The Limit Order are matched based on amount and time. The orders are listed Highest to Lowest on the Buy Side. The orders are listed Lowest to Highest on the Sell Side. If there are 2 Sell orders for same amount the order which is first in time [fractions of milliseconds] is first.  The about is the example as to how the orders would look like on any exchange.  Now the highest price the buyer is ready to pay is 20.21 and the lowest price a seller is ready to sell for is 20.25. Hence there is no trade. Now if a new Buy order comes in at 20.25, it matches with the sell and the deal is made. If a new Buy order comes in at 20.30, it still matches at 20.25. Similarly if a Sell order come in at 20.21, it matches and a deal is made. If a Sell order come in at 20.11, it still matches 20.21.   Incase of market order, with the above example if there is a Buy order, it would match with the lowest sell order at 20.25, if there is not enough quantity , it would match the remaining quantity to the next highest at 20.31 and continue down. Similarly if there is a Sell market order, the it would match to the maximum a seller is ready to buy, ie 20.21, if there is not sufficient buy quantity at 20.21, it will match with next for 20.19   If say there are new buy order at 20.22 and sell orders at 20.24, these will sit first the the above queue to be matched. In your above example the Lowest Sell order was at 20.10 at time t1 and hence any buy order after time t1 for amount 20.10 or greater would match to this and the price would be 20.10. However if the Buy order was first ie at t1 there was a buy order for 20.21 and then at time later than t1, there is a sell order for say 20.10 [amount less than or equal to 20.21] it would match for 20.21. Essentially the market looks at who was the first to sell at lower price or who was the first to buy at higher price and then decide the trade.  Edit [To Clarify xyz]: Say if there is an Sell order at $10 Qty 100. There is a buyer who is willing to pay Max $20 and is looking for Qty 500. Your key assumption that the Buyer does not know the current SELL price of $10 is incorrect. Now there are multiple things, the Buyer knows the lowest Sell order is at $10, he can put a matching Buy order at $10 Qty 100, and say $11 Qty 100 etc. This is painful. Second, lets say he puts a Buy order at $10 Qty 100, by the time the order hits the system someone else has put the trade at $10 and his order is fulfilled. So this buyer has to keep looking at booking and keep making adjustments, if its a large order, it would be extremely difficult and frustrating for this Buyer. Hence the logic of giving preference. The later Buy order says ... The Max I can pay is $20, match eveything at the current price and get the required shares. A stock is only worth what someone is willing to pay for it. If it trades different values on different days, that means someone was willing to pay a higher price OR someone was willing to sell at a lower price.  There is no rule to prevent a stock from trading at $10 and then $100 the very next trade... or $1 the very next trade. (Though exchanges or regulators may halt trading, cancel trades, or impose limits on large price movements as they deem necessary, but this is beside the point I'm trying to illustrate). Asking what happens from the close of one day to the open of the next is like asking what happens from one trade to the next trade... someone simply decided to sell or pay a different price.  Nothing needs to have happened in between. "During the day, market and limit orders are submitted at any time by market participants and there is a bid and an ask that move around over time.  Trades occur whenever a market order is submitted or a limit order is submitted that at a price that matches or exceeds an existing limit order.  If you submit a market order, it may consume all best-price limit orders and you can get multiple prices, changing the bid or ask at the same time.  All that stuff happens during the trading day only. What happens at the end of the day is different.  A bunch of orders that were submitted during the day but marked as ""on close"" are aggregated with any outstanding limit orders to create a single closing price according to the algorithm established by the exchange.  Each exchange may handle the details of this closing event differently.  For example, the Nasdaq's closing cross or the NYSE's closing auction. The close is the most liquid time of the day, so investors who are trading large amounts and not interested in intraday swings will often submit a market-on-close or limit-on-close order.  This minimizes their chance of affecting the price or crossing a big spread.  It's actually most relevant for smaller stocks, which may have too little volume during the day to make big trades, but have plenty at the close.  In short, the volume you see is due to these on-close orders. The spike in volume most likely has no special information about what will happen overnight or the next day.  It's probably just a normal part of the market for illiquid stocks." You should not look at volume in isolation but look at it together with other indicators and/or the release of news (good or bad). When there is lower than average volume this could be an indication that the stock is in a bit of a holding pattern, possibly waiting for some company or economic news to come out (especially when accompanied by small changes in price). It could also mean that trading in a certain direction is drying up and the trend is about to end (this could be accompanied with a large move in price). When there is higher than average volume (2 to 3 times more or higher), this could be due to the release of company results, company or economic news, or the start or end of a trend (especially if accompanied by a gap). A large increase in volume accompanied by a large fall in price (usually a gap down) may also be an indication the stock has gone ex-dividend. There could be a range of reasons for variations in volume to the average volume. That is why you need to look at other indicators, company reporting and news, and economic news in combination with the volume changes to get a grasp of what is really happening. Making or losing income (via selling shares) is the taxable event, not moving the income you made to and from an account.  The only exception would be a special account such as an IRA, and then there would be rules specific to that account structure about when you can withdraw money and what the tax consequences are.