The formation of secondary products in plant tissue cultures is reviewed. The conditions for the enhanced production of secondary products, which include alkaloids, terpenoids, steroids and phenolics, can be regulated in a number of ways. For example, manipulation of secondary product formation is possible by varying the nutrient composition of the growth medium, light, temperature and pH, and by the use of elicitors, permeabilisation and two-stage systems. Molecular engineering and the use of biomass and large-scale culture are described along with future prospects for the commercial production of secondary products from cell suspension cultures.