Abstract: Phong shading is one of the best known, and at the same time simplest techniques to arrive at realistic images when rendering 3D geometric models.However, despite (or maybe due to) its success and its widespread use, some aspects remain to be clarified with respect to its validity and robustness. This might be caused by the fact that the Phong method is based on geometric arguments, illumination models, and clever heuristics. In this article we address some of the fundamentals that underlie Phong shading, such as the computation of vertex normals for nonmanifold models and the adequacy of linear interpolation and we apply a new interpolation technique to achieve an efficient and qualitatively improved result.