Tangential endwall contouring (TEWC) is intended to improve the turbomachinery blading efficiency. This paper summarizes the experimental and numerical investigation of a test turbine with endwall contoured vanes and blades. Constant section (2D) airfoils as well as optimized compound lean (3D) high pressure steam turbine blading in baseline and endwall contoured configurations have been examined. Brush seals (BSs) are implemented within the casing sided cavities to minimize the leakage flow near the tip endwalls, where the contouring is also applied. The pressure and temperature data that are recorded in three axial measuring planes are plotted to visualize the change in flow structure. This shows that the efficiency is increased for 2D airfoils by means of endwall contouring. However, the efficiency of the first stage suffers, and the endwall contouring is still beneficial for the overall performance of the engine. Both phenomena (an efficiency loss in stage one and an improvement of the performance in stage two) have also been measured for the optimized 3D configurations; thus, it can be expected that the endwall contouring has also a beneficial impact on the performance of multirow turbines. The numerical investigations demonstrate in detail, how the secondary flow phenomena are influenced by end-wall contouring and a description of the changes in vortex formations as well as blade loading are given for the various configurations. It has been found that for steady computational fluid dynamics (CFD) simulations the use of stage interfaces suppresses the positive effects of the endwall contour onto the downstream blade row.