Free energy relationships are a ubiquitous means of characterizing trends in rates of reaction with changing molecular structure. They may be used to quantify the extent of progress along a reaction coordinate at a reaction's transition state or alternatively the extent of similarity between a reaction's transition state and some reference transformation. This critical review outlines correlative procedures for the treatment of experimentally-determined free energy relationships with a particular focus on enzyme-catalyzed group transfers. The reasons for observed non-linearities in free energy relationships are considered. Attention is paid to the influences of changes in kinetic mechanism (e. g. general-acid catalyzed versus uncatalyzed reactions, and the competition between associative, dissociative and concerted modes of group transfer), changes in rate-determining step and the choice of an appropriate reference reaction. The relationship between experimental data and physical/theoretical models of reactivity is discussed (191 references).