The paper examines the behaviour of a pitching-and-surging wave-energy converter driven by unidirectional waves that exert harmonically varying forces on its hull. The power take-off (PTO) is by means of an inertia that moves either on a straight horizontal rail or on an arm that turns about a horizontal axis. Angular displacements are taken to be small, so that linear analysis is appropriate; these are idealized conditions, but any wave-energy converter must be able to operate effectively in ideal conditions. For good power capture with the least engineering difficulties, it is found that the power take-off inertia should be centred as high as possible above the overall centre of mass G. It is shown that power is captured from the waves by motion of the centre of pressure P relative to the overall centre of mass G. This can only result from motion in pitch and is dependent on the fact that there is a large vertical distance between points P and G. It is found in practical cases that the added mass of water generally brings G closer to P, and this limits the power that can be captured.