The intention was to explain how methods using Fourier transformation and complex analysis lead to sharp regularity results in the study of fractional-order operators such as (−∆) a , the fractional Laplacian (0 < a < 1), and to give an overview of the results. As required by the organizers, we start at a fairly elementary level, introducing the role of function spaces and linear operators. In the later text we explain two important points in detail, with an elementary argumentation: How the exact solution spaces (the a-transmission spaces) come into the picture, and why a locally defined Dirichlet boundary value is relevant.Here is a small selection of the many contributors to the field: Blumenthal and Getoor [BG59], Vishik and Eskin '60s (presented in [E81]), Hoh and