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In internal combustion engines, variable valve timing (VVT) is the process of altering the timing of a valve lift event, and is often used to improve performance, fuel economy or emissions. It is increasingly used in combination with variable valve lift systems. There are many ways in which this can be achieved, ranging from mechanical devices to electro-hydraulic and tubeless systems. Increasingly stringent emissions regulations are causing many automakers to use VVT systems. Valves within an internal combustion engine are used to control the flow of the inlet and exhaust gases into and out of the combustion chamber. The timing, duration and elevation of these valve events has a significant impact on engine performance. Without variable valve timing or variable valve lift, valve timing must be the same for all engine speeds and conditions, so compromises are required. An engine equipped with a variable valve timing drive system is freed from this limitation, allowing improved performance over the engine operating range.

Piston motors normally use valves that are driven by camshafts. The cams open the valves (lift) for a certain time (duration) during each intake and exhaust cycle. The opening and closing moment of the valve is also important. The camshaft is driven by the crankshaft through timing belts, gears or chains. A motor requires large amounts of air when operating at high speeds. However, the intake valves can be closed before there is enough air in each combustion chamber, which reduces performance. On the other hand, if the camshaft keeps the valves open for longer periods of time, such as with a racing cam, problems begin to occur at lower engine speeds. This will cause the unburned fuel to exit the engine as the valves are still open.