Electron spin resonance has become a major tool for investigating biological one-electron or radical group transfer. The scope and limitations of this method are considered and emphasis is placed on the first of the following two questions that govern the field: (1) What is the structure, stability and potential biological function of radicals that might occur as biological intermediates? (2) Which radicals have been demonstrated up to now as (a) occurring in biological reactions, (b) being essential biological intermediates? The two questions deserve independent consideration and supplement each other. However, the second question can hardly be decided before the first one, though there may be a severe temptation to claim the occurrence or even stabilization of a certain radical without any structural evidence. Among the free radicals considered here are phenoxyls, mercaptyls, semidiones, (aza)semiquinones (from flavins and pteridines), and metal-stabilized radicals.