This paper gives an overview of existing knowledge concerning the structure and deduced functions of polyketide synthases active in antibiotic-producing streptomycetes. Using monensin A as an example of a structurally complex polyketide metabolite, the problem of understanding how individual strains of microorganism are 'programmed' to produce a given polyketide metabolite is first outlined. The question then arises, how is the programming of polyketide assembly related to the structural organization of individual polyketide synthase complexes at the biochemical and genetic levels? Experimental results that help to illuminate these relations are described, in particular, those giving information about the structures and deduced functions of polyketide synthases involved in aromatic polyketide biosynthesis (actinorhodin, granaticin, tetracenomycin, whiE spore pigment and an act homologous region from the monensin-producing organism), as well as the macrolide polyketide synthase active in the biosynthesis of 6-deoxyerythronolide A.