A theory of temperament is presented which draws upon the tradition of neo-Pavlovian typology of the nervous system. Special attention is paid to temperament as one of the regulators of the relationship between humanity and environment, when the environment is treated as a source of stimulation. The main focus is upon temperamental characteristics associated with behaviour at the energetics level. This comprises two dimensions: reactivity and activity. A theoretical analysis of the two traits is conducted and their relationship with human behaviour is elaborated. A number of studies are discussed, illustrating the role of reactivity as one of the regulators of the individual's style of action and as a partner in the determination of preferences for situations and for behaviours of a given stimulatory potential. Finally, the paper provides a number of data exemplifying essential relations between temperament and personality. The interdependencies between both these phenomena are discussed in the light of the assumption that temperament should be regarded as a consequence of biological evolution and personality as a product of the social environment.