Electron Spin Resonance (ESR) spectroscopy is the technique sine qua non for establishing the geometric and electronic structures of the free radicals participating in many organic reactions and syntheses. It also allows the mechanisms of the complex network of elementary reactions constituting the overall chemical change to be determined and the rate parameters of these reactions to be measured. Radicals can be classified into two main types - and π-radicals. -radicals, exemplified by phenyl, have their free electron orbital projecting out from a non-planar radical centre. They are usually more reactive than -radicals, exemplified by alkyl radicals in which the free electron resides in a carbon 2pz orbital perpendicular to the planar radical centre. Fluorine substitution results in a progressive change from a planer -methyl radical to a tetrahedral -trifluoromethyl radical. The regioselectivity and stereoselectivity of intermolecular and intramolecular free atom and radical additions to unsaturated systems are discussed in terms of optimal orbital overlap, steric and free valence effects as are group transfers and SH 1 and SH 2 homolytic substitution reactions. The unusual reactions of aluminium and gallium atoms with alkenes, alkynes 1,2 and 1,3 dienes and benzene are considered. Recent controversies concerning the Fenton reaction and the free radical mode of action of the new antimalarial drug Artemisinin are reviewed. The potential for applications of ESR in conjuction with spin traps in heterogeneous catalysis is outlined.