This chapter discusses the frequency-domain modelling approach to model wave energy converters (WECs), and presents its advantages and limitations. It is shown that frequency-domain modelling requires the linearization of the forces acting on the WEC. This simplification is acceptable for waves and small-amplitude device oscillatory motions and whenever the mooring system can be modelled by a linear spring and the power take-off (PTO) using either a linear damper or a linear spring-damper system. Under these circumstances, the first step to model WECs is typically based on a frequency-domain approach, where the excitation is assumed to be of a simple harmonic form. Accordingly, all the physical quantities vary sinusoidally in time with the same frequency of the incident wave. Therefore, in the frequency domain the equations of motion become a system of algebraic linear equations that may be solved straightforwardly. However, this requires the calculation of the hydrodynamic coefficients, which can be produced relatively easily using a linear potential flow solver and boundary element methods (BEMs). Hence, frequency-domain models are relatively fast and so widely used to get a first insight on response and power capture of WECs. Nevertheless, despite the advantages of frequency-domain models, it is important to take into account that these models may be relatively inaccurate for large waves, at frequencies close to resonance or where viscous forces are significant.