This chapter gives an overview of the development of wave energy converters (WECs) from initial conception to commercial demonstration. The structure of the chapter follows the Technology Readiness Level (TRL) approach, as recommended by several documents for the development of WECs. An overview about the TRL approach is given in Section 8.04.1. The development of a device requires substantial fiscal support and funding opportunities. These opportunities, focusing on the UK, are reviewed in Section 8.04.2, together with further benefits for device developers as a result of funded activities. Physical model testing in the laboratory is an essential part in the research and development of WECs. Section 8.04.3 addresses those phases of TRLs (TRL 1–3) taking place in the laboratory environment. The first part of this section addresses similarity theory and scale effects and shows how the results in the model can be up-scaled to full scale. The second part describes the design and testing of physical scale models including topics such as test facilities, wave generation methods, model design, measurement equipment, or testing the device under different wave conditions. In contrast to tests under laboratory conditions, the ocean environment is not controllable and investigations are much more expensive and time consuming. Such tests taking place in the sea (TRL 3–5) are covered in Section 8.04.4. Tests conducted at a benign test site, which are still based on a scaled version of the device, are covered first. Then some information about sea trials based on full-scale prototypes is reviewed. The last part of this section addresses sea trials with WECs in arrays. Irrespective of the TRL test phase, there exists two basic approaches to analyze the measured data namely frequency domain and time domain analysis. These two approaches are described in the final Section 8.04.5 of this chapter.