In guiding the progression and implementation of wave energy converters in a more effective and solid way, stepwise protocols have been recommended for assessing and validating their performance, feasibility, reliability and survivability during the devices׳ progression stages from the concepts to full-scale commercial devices. One important aspect is scale model testing in different development stages as a path to solve the most important problems and to build confidence in the device development. Particularly, in the early development stages of the wave energy converters, small scale models are often tested in well-controlled laboratory conditions in a manner that some dynamic effects can be isolated, hence the analysis and understanding of the dynamic process could be much simplified and specified. However, there is no theory or guideline developed for this scaling practice in explaining whether or not the scaling is correct and how the test data can be used. In this paper, a theoretical analysis to the requirements and an explanation to the feasibilities of physical modelling/scaling, and some important scaling issues on physical modelling of wave energy converters, are presented with an emphasis on the physical modelling and scaling of power take-off systems. This theoretical analysis can help to understand why and how a small scale model can be tested and how the test data can be used.