Generally, most airlines allow you to carry on one bag plus one personal item per passenger.
Your bag should meet the following requirements:
Does not exceed 45 inches when you total length plus width plus height.
Weighs less than 40 pounds (18kg.)
Fits in an overhead bin or underneath the seat in front of you.
Some examples of personal items:
Purse, briefcase, laptop computer, camera bag, diaper bag or other personal item of similar size.
These are general guidelines. You should always check with your airline or visit their website to determine if your carry on luggage meets their requirements.
For more information on baggage requirements click on the link below to visit the Federal Aviation Administration website:
For additional information on what you can bring on board click on the FAA link below:
American Airlines http://www.aa.com/content/travelInformation/baggage/baggageAllowance.jhtml
US Airways http://www.usairways.com/awa/content/traveltools/baggage/baggagepolicies.aspx
Jet Blue http://www.jetblue.com/help/topics/help_topic_baggageinfo.html