It is exactly 100 years since Moses Gomberg claimed that he had observed a substance containing a trivalent carbon atom i. e. a carbon-centred 'free radical' (nowadays, simply a carbon-centred 'radical'). The subsequent development of radical chemistry was at first very slow, but blossomed with the development of synthetic polymers, especially during and after World War II. In what is now generally understood by radical chemistry we are dealing with reactive, short-lived species which are electrically neutral. By the late 1960s, the essential features of the subject were well understood, and quantitative data on the energetics and kinetics of reactive radicals were rapidly accumulating. This short text sets out to present that basic understanding in a modern context, in which extensive use of radical reactions is now being made in organic synthesis, and where, in the life sciences, reactive radicals are being recognised both as mediators of many disease conditions, and frequently as key players in mechanisms of enzyme action.