Desmodromic valvegear is opened and closed by the action of a cam.

In conventional valvegear each valve is lifted from its seat by the cam, compressing the valve spring. As the cam continues to turn and passes its maximum lift point, the spring returns the valve to its seat. This system is simple and effective, but has the drawback that at very high speeds the spring cannot act fast enough, and the rate at which the valve closes lags behind the optimum determined by the closing ramp of the cam.

In desmodromic valvegear a second cam lobe, with a very different profile, is used in conjunction with a lever system to positively close the valve.

Desmodromic valvegear has been used in racing engines in cars (notably Mercedes sports and Grand Prix cars in the 1950s) and motorcycles (Ducati). Because it eliminates the possibility of valve bounce, desmodromic valvegear allows higher engine speeds which can lead to greater power output.

The drawbacks are cost, complication, noise and potentially greater maintenance.

Modern techniques for calculating cam profiles, which make the simpler spring-assisted valvegear more reliable at high speeds, have made desmodromic valvegear less attractive.