Here in Seattle we know what it's like to need the A/C in the summer. It's especially nice to have when getting back into the car on a hot August Day.
A vehicles Air Condition System relies on a Gas known as R-134a (1,1,1,2-Tetrafluoroethane) or in older vehicles R-12 (Dichlorodifluoromethane). R-134a replaced R-12 because it less harmful to the environment than r-12, but is less effective at cooling your car. This gas pumped and circulated through many different areas in your vehicle and under the hood to make your A/C work. There are many pipes and many connections on those pipes to route the gas where it needs to go. Each of these pipes and connections have to have rubber o-rings to seal the gas from escaping. After time the gas naturally dissipates and also these rubber seals wear out and cause the gas to escape, leaving you with a non-functioning A/C.
The first step to diagnosing an A/C problem is to identity if there are any leaks. You can do this by looking over all the lines and fittings for any fluid build-up or also insert a die into the gas and use a U.V. light to watch it come out of the leaking area. At this point you can replace the o-ring, pipe, or leaking component, fill the system and be back in the cool air.