When an HDD is powered up, the disks are spun to a precisely regulated speed and the heads are allowed to move radially over the disk surfaces. Limited vertical movement within a very small range, self-regulating by the formation of an air bearing surface (ABS) between the head and slider is also allowed. Accurate and precise control of radial position of the head slider is done by the head positioning servomechanism. This servomechanism is a feedback system consisting of a sensing element that measures the displacement of the head, a servo motor and actuator, an amplifier, and a controller controlling the movement of the actuator. In the early generations of HDD, the controller used to be implemented using analog electronics but all modern drives come with digital controller. Nowadays almost all practical servomechanisms, HDD head positioning servo being one of them, use µ-controller or digital signal processor (DSP) to implement the controller. In this chapter, functions and principle of operations of different components of the HDD servomechanism are discussed, sources of noise and disturbances are explained, and the basic guidelines for design of controller are presented.