The last few decades have seen significant progress in our understanding of the occurrence of magnetic fields in stars with radiative envelopes, in particular in massive stars and in intermediate mass stars at early evolutionary stages. This book provides a comprehensive review of the most recent achievements in the measurements of stellar magnetic fields in O, B, and A stars. These include the archetypes of stellar magnetism, the chemically peculiar Ap and Bp stars, O- and early B-type stars with their magnetospheres, accreting Herbig Ae-Be stars, Wolf-Rayet stars, and high-mass X-ray binaries, among others. It provides an overview of the underlying physics for the interpretation of the data and identifies the requirements, both observational and theoretical, for improving our understanding of the origin of the magnetic fields in early-type stars. It aims to educate scientists working on stars, who are not yet experts in magnetic field studies, assuming that the reader is already familiar with basic terms and concepts of stellar astrophysics.