This paper discusses the restrictions on frequency domain modelling of wave energy converters (WECs). It is shown that, for a model where the radiation is represented as causal, and where the control signal is not acausal, a frequency domain model is suitable for finding the post-transient, linear, causal response of a WEC. The common use of models that do not represent the casual nature of radiation (memory), or that include an acausal control signal, are identified in the literature. Arguments are presented for restricting both models to sinusoidal motion. Correct and incorrect applications of these restricted models to numerical and experimental work are discussed, with examples given from literature. Several papers are identified that could be interpreted as stating that frequency domain modelling of WECs is restricted to sinusoidal motion only. However, it is shown that these papers are in fact discussing the limitations of models where the control signal, or the WEC itself, are represented as mass-spring-damper systems.