Chapter 5 is mainly devoted to the interaction between waves and immersed bodies. In general, an immersed body may oscillate in six different modes, three translating modes (surge, sway, heave) and three rotating modes (roll, pitch, yaw). An oscillating body radiates waves, and an incident wave may induce a corresponding excitation force for each one of the six modes. When a body oscillates, it radiates waves. Such radiated waves and excitation forces are related by so-called reciprocity relationships. Such relations are derived not only for a single oscillating body but even for a group (or 'array') of immersed bodies. Axisymmeric bodies and two-dimensional bodies are discussed in separate sections of the chapter. Although most of this chapter discusses wave-body dynamics in the frequency domain, a final section treats an immersed body in the time domain.

Referring to a simple illustration, a verbal explanation is given by the essential, but perhaps paradoxical, statement that to absorb wave energy from a wave by means of an oscillating system, it is required that the system radiates a wave which interferes destructively with the incident wave. Then various mathematical relations are derived concerning the conditions for an oscillating body to remove energy from an incident wave. The mathematical conditions for wave-power absorption may be illustrated as a paraboloid-shaped 'island' on an infinite complex-plane 'ocean' surface. The top of this 'island' corresponds to maximum absorbed power. An additional matter is the optimum control of a wave-energy converter (WEC) body. Thus far, the WEC body's shape and oscillation mode have been taken into account, but not its physical size. The latter is an important parameter related to the cost of the WEC, when the Budal upper bound is explained and discussed. Another important phenomenon, related to the Keulegan–Carpenter number, is discussed, in relation to an example of a WEC body. In a final section of the chapter, a WEC body, oscillating in several modes of motion, is discussed.