Airline Apps That Check You In, Map Airports and Follow Luggage NOW that half of all travelers carry smartphones, airlines are rolling out apps that allow these devices to take care of most of the tasks agents used to handle. While many travel apps specialize in things like tracking a flight (FlightStats) or guiding travelers through airports (GateGuru), the airline apps aim to do it all: from checking in to flight status updates and baggage tracking. American and Delta offer apps for the broadest range of devices: the iPhone, BlackBerry, Android and Windows phones. Southwest has apps for all of them except Windows, and United and Continental offer apps for iPhones and Androids. JetBlue released its first iPhone app on Feb. 6. Separate apps are in the works for the iPad and other tablets. Depending on the function, some apps are better than others, but all are free, and most travelers will find them to be useful tools, particularly on the day of travel. Here are a few of the ways you can use airline apps. CHECKING IN Although you could use your phone's Web browser to check in, all the new airline apps are designed to make the process quicker. Once you log in with your frequent flier number or flight confirmation, the app locates your itinerary and you can check in with a few taps. That's especially useful if you're flying Southwest, which assigns boarding priority based in part on how early passengers check in (starting 24 hours before flight time). "Once you check in, you're assured a spot in line, so that feature on our app is very important," said Whitney Eichinger, a Southwest spokeswoman. Most airline apps (except Southwest's) let you view and change your seat assignment if a better seat opens. Delta, United and American also let you monitor your standby status (Delta and United track upgrades, too) and save a digital boarding pass on your phone. That last option has met with mixed reviews among travelers - myself included - since airport scanners sometimes have trouble reading the bar code on mobile boarding passes. But even if you print a boarding pass later, an app lets you check in 24 hours before your flight, no matter where you are. Before a recent Delta flight, the airline's app even popped up an alert on my iPhone reminding me it was time to check in. TRACKING A FLIGHT One of the most useful features the airlines apps offer is a way to track flights, although some carriers are still working on proactively sending alerts to the phone's home screen rather than making customers navigate through the app to find out whether a flight is on time. Still, using an app is more convenient than calling the airline's automated system to check a flight's status; in some cases, the app gives you more information. On the way to the airport before my Delta flight, I used the flight status feature on Delta's app and could see that even though my flight was listed as "on time," the incoming plane was running late. I figured that meant my flight would take off late (which it did), and decided there was time to run an errand on the way to the airport. Airline apps are especially handy when you have a connection: once your first flight lands, you can look up the gate number and status of your next flight while you're taxiing to the terminal, rather than waiting until you get off the plane and find an electronic billboard. Although most airlines let customers sign up for e-mail or text alerts about flight delays, these notifications can be hit or miss (depending on how often the carrier decides to send them). JetBlue's new app sends flight alerts to the iPhone's home screen, saving passengers the extra step of looking for an update; other airlines say they're working on similar "push" notifications. MONITORING YOUR LUGGAGE Delta is ahead of the pack in offering a novel feature on its app: the ability to track your checked bag, much as you can a FedEx package. Once you check your bag and get a receipt at the airport, you can select the "Track My Bags" feature on Delta's app, which will guide you through using your phone's camera to scan the bar code on your baggage receipt (or you can enter the bag tag number instead). From there, the app lets you see where your bag is within Delta's system. It will show that the bag has been received for check-in, inducted into the baggage system, scanned planeside or is at baggage carousel No. 4," said Paul Skrbec, a Delta spokesman. You can also use Delta's app to pay for your bag if you check in with your phone, then drop off your luggage and get a receipt once you get to the airport. NAVIGATING THE AIRPORT Another useful feature airline apps offer is airport maps, and for now United is the leader. Its app includes detailed maps for more than 100 airports, and that alone makes it worth downloading, even if you're not flying United. Once you select an airport, you can zoom in on the map to see not only where various gates are, but also where you can find baggage claim locations, airline clubs, rental car counters, a Starbucks, a bookstore and other retailers and restaurants. American offers maps for just six airports, and they only display gate numbers, not the amenities available, although Matt Miller, an American spokesman, said that an app update, which is due in the coming months, would include detailed maps for more than 80 airports. Delta's 26 maps are relatively basic - they don't show retailers or specific gate numbers - but Mr. Skrbec said improvements are in the works. Henry Harteveldt, a travel analyst and co-founder of the Atmosphere Research Group, expects that airlines and airports will ultimately use these mapping tools and the devices" geo-targeting capabilities to send special offers to travelers - like a coupon for a discount at a nearby store. Likewise, he sees an opportunity for airlines to use their apps to manage the rebooking process when passengers miss a connection, perhaps by displaying a special phone number customers can call to get expedited help. For now, he said, booking and rebooking are tasks most passengers prefer to do on a larger screen or over the phone, although some airlines do offer booking via their apps. "We're still in Phase One of all this," Mr. Harteveldt said. When it comes to mobile applications, the travel industry collectively is still in the crawl stage.