Magnetic fields occur in every part of the Hertzsprung-Russell Diagram, in single and binary hot and cool stars both on and off the main sequence, with profound consequences for internal stellar structure, rotational evolution, and circumstellar environments. In addition to direct magnetic measurement via spectropolarimetry, the presence of magnetic fields can often be inferred from indirect photometric or spectroscopic diagnostics. We review the techniques used to measure stellar magnetic fields, the general properties of magnetic fields in different classes of stars, and outline some of the open problems, in particular as regards fossil magnetism in hot stars. This article serves as the context for an overview of the highlights of the BRITE mission and the BRITEpol spectropolarimetric survey, including newly detected magnetic stars, and new information on the photometric variability of magnetic stars.