Naturally occurring polyhydroxyalkanoates (PHAs) are optically active linear polyesters with each repeating unit in the stereochemical R-configuration. Their physical and mechanical properties are largely determined by the chemical structure and relative amount of the monomers, as well as the molecular weight. To date, more than 150 different monomeric constituents have been reported in microbially synthesized PHAs. As a result, properties may vary. In general, SCL-PHAs are thermoplastics with high melting temperature and crystallinity. MCL-PHAs are typically elastomers with low melting temperature and crystallinity but good elongation properties. SCL-co-MCL-PHAs are somewhere in between depending on the exact compositions. Physical and mechanical properties of selected typical PHAs are listed in the following table.