To put it simply, a transmission brings the power from the engine to the drive wheels through a series of gear sets and friction devices. In an automatic transmission -- the most popular type in the U.S. -- it also has a brain called the valve body. The valve body responds to electrical and hydraulic signals. Nearly all of the vehicles on the road today use a complex system of computers, switches and solenoids to perform various tasks, including when to tell the transmission to change gears. Although it may sound simple, the transmission is the most complex component in your automobile. It may have as many as 1,000 parts, each which has to function perfectly with the others in order to work properly.