The chapter provides an introduction to state of the art electron paramagnetic resonance (EPR) techniques as they are applied to the study of biological systems. Particular emphasis will be placed upon elucidating some of the problems encountered in the study of real systems and upon evaluating some of the newer instrumental methods. Philosophically, applications are divided into the study of systems containing naturally occurring paramagnetic centers and those that are naturally diamagnetic but which have been labeled by a spin-containing probe. The latter type of study is well suited to the systematic application to a great many biological problems and thus will receive much attention. Hydrodynamic characterization of biomolecules involves the determination of either the translational or rotational diffusion rates. The rotational diffusion coefficient depends on the third power of the molecular radius as compared to the linear dependence observed for translational diffusion. McConnell and co-workers contributed much to the theoretical understanding of the EPR spectra of spin labels.