Wave Energy Converters (WECs) extract energy from ocean waves and have the potential to produce a significant contribution of electricity from renewable sources. However, large "WEC farms" or "WEC arrays" are expected to have "WEC array effects", expressed as the impact of the WECs on the wave climate at an installation site, as well as on the overall power absorption of the WEC array. Experiments have been performed in the Shallow Water Wave Basin of DHI (Denmark) to study such "WEC array effects". Large arrays of up to 25 heaving point absorber type WECs have been tested for a range of geometric layout configurations and wave conditions. WEC response, wave induced forces on the WECs and wave field modifications have been measured. Each WEC consists of a buoy with diameter of 0.315 m. Power take-off is modeled by realizing friction based energy dissipation through damping of the WECs' motion. This unique experimental set-up of up to 25 individual WEC units in an array layout, placed in a large wave tank, is at present the largest set-up of its kind studying the important WEC array effects. The data obtained from these experimental tests will be very useful for the understanding of WEC array effects and for validation and extension of numerical models. This model validation will enable optimization of the geometrical layout of WEC arrays for real applications and reduction of the cost of energy from wave energy systems.