The modelling approach presented in this chapter is that of system identification, where models are determined from input/output data measured from the system under study. Models identified from recorded wave energy converter (WEC) data can accurately describe WEC behaviour, provided the data is of a sufficiently high quality. The chapter details generating data for the system identification process and outlines the requirements of the data to ensure representative models are obtained. Different parametric model structures are presented, along with identification algorithms to determine the optimum parameter values for the models. A number of case studies illustrating the use of system identification to obtain numerical models of WECs are given.