This review discusses the evolution of the chemical composition of the molecular gas throughout the process of star formation. It covers the initial stages of cloud-core formation, gravitational collapse and the formation of the star, the effects of stellar radiation and outflows on the surrounding cloud, and the composition of the circumstellar disk. A brief overview of relevant chemical processes sets the stage. The ensuing evolution of the chemistry is governed by several competing timescales and by energy input by the young star(s). Special attention is given to the role of molecular freeze-out on dust grains and to deuterium fractionation in cold gas.