These are broadly of two types semiautomatic and fully automatic ones. In the first type only the clutch is operated automatically, the driver still has to select the gears. In the later type which is employed in modern cars, even the gears are changed automatically by a control mechanism which is actuated by the accelerator pedal only. In the cars with fully automatic transmission, there are only two pedals, for breaking and for acceleration. In addition there is a selector lever.

The present day automatic transmissions consists of multi plate clutches, torque converter and epicyclic gearbox.

Automatic transmissions operate basically by controlling vehicle speed and engine load. Increasing vehicle speed needs changing gears upwards, whereas increasing engine load necessitate change from higher to lower gears. For the speed factor a sensitive control unit (mechanical, hydraulic or electrical) is driven off a suitable point beyond transmission and thus its control action becomes sensitive to vehicle speed. The engine load factor is introduced by the position of the accelerator pedal. Thus the engine load sensitive control unit is operated by the depression of the accelerator pedal.

In the modern automatic transmissions, the complicated hydro mechanical control systems have been replaced by electronic control, which gives better response and easier gear changes. The computerized controls enable vehicles to change gears just like a skilled driver.