says that y is the time integral of x. This mathematical operation occurs in the central nervous system and is the subject of this review. The review concentrates on the integrator of the vestibulo-ocular reflex as a prominent example, offers a model of how integration might be done by neurons, and speculates about the extent to which neural integrators occur elsewhere in motor control. Integration describes physical processes all around us: The volume of a fluid (blood) in a container (ventricle) is the integral of the inflow (venous return); the position of the shaft of a d.c. motor is the integral of the current applied to its armature. These examples, however, are just statements of physics. They are not examples of devices deliberately constructed by nature or technology to integrate a signal to achieve some desired end. Such devices are not very visible in our world. One exception is attached to the back of our houses, it measures the energy we use by integrating our power consumption, but most integrators hide in boxes that operate cranes, fly airplanes, orient satellites, and so on. In these examples, the integrators are usually located in negative feed­ back control systems. Their value there, to oversimplify, is that integrators have very large gains at low frequencies, making controllers very accurate