Strain Wave Gearing (also known as "harmonic drive") is a special type of mechanical gear system that can improve certain characteristics compared to traditional gearing systems (such as Helical Gears or Planetary Gears). It was invented in 1957 by C.W. Musser. The advantages include: no backlash, compactness and light weight, high gear ratios, reconfigurable ratios within a standard housing, good resolution and repeatability when repositioning inertial loads, high torque capability, and coaxial input and output shafts. High gear reduction ratios are possible in a small volume (a ratio up to 320:1 is possible in the same space in which planetary gears typically only produce a 10:1 ratio).

Disadvantages include a tendency for 'wind-up' (a torsional spring rate) and potential degradation over time from mechanical shocks and environment.

They are typically used in industrial motion control, robotics and aerospace, for gear reduction but may also be used to increase rotational speed, or for differential gearing.