While the biological functions of most of the secondary metabolites made by streptomycetes are not known, it is inconceivable that they do not play an adaptive ecological role. The biosynthesis of secondary metabolites under laboratory conditions usually occurs in a growth phase or developmentally controlled manner, but is also influenced by a wide variety of environmental and physiological signals, presumably reflecting the range of conditions that trigger their production in nature. The expression of secondary metabolic gene clusters is controlled by many different families of regulatory proteins, some of which are found only in actinomycetes, and is elicited by both extracellular and intracellular signalling molecules. The application of a variety of genetic and molecular approaches is now beginning to reveal fascinating insights into the complex regulatory cascades that govern this process.