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Medium-chain-length polyhydroxyalkanoates (mcl-PHAs) are bacterial polyesters which are produced in nature by certain Pseudomonas strains. These biopolymers are of interest because of their chirality, biodegradability and elastomeric property, while the 3-hydroxy acid monomers are a potential source of chiral synthons. Since wild-type bacteria can only be used to a limited extent for the production of tailor-made functionalized PHAs, various recombinant bacteria have been generated in order to elucidate the PHA synthesis pathway and to control and modify the metabolic carbon flux towards PHA synthesis. Production of PHAs with altered monomer composition and physical properties has already been achieved by pathway engineering. An alternative to the in vivo synthesis strategies using whole microorganisms is PHA synthesis in vitro with isolated enzymes, which is of interest for incorporation of specific monomers that are not taken up or metabolized by bacterial cells. The various synthesis strategies are discussed in the context of the possible future production of tailor-made functionalized PHAs.