During the decade and a half that has elapsed since the intro duction of principal functions (Sario 8 J), they have become impor tant tools in an increasing number of branches of modern mathe matics. The purpose of the present research monograph is to systematically develop the theory of these functions and their ap plications on Riemann surfaces and Riemannian spaces. Apart from brief background information (see below), nothing contained in this monograph has previously appeared in any other book. The basic idea of principal functions is simple: Given a Riemann surface or a Riemannian space...