In the academic year 1994/95 I lectured on semigroups in the Scuola Normale Superiore and in the Politecnico di Torino. The purpose of the lectures was to present to an audience of graduate students a self-contained introduction to the theory of strongly continuous semigroups of linear operators acting on a Banach space. The main trust was concentrated on laying down the basic geometrical aspects of the theory as a background for applications to concrete problems in the analysis of differential operators.