Polyhydroxyalkanoates (PHAs) serve numerous bacteria as storage compounds. It is generally believed that under unbalanced growth conditions, n-hydroxyalkanoates are synthesized inside the bacterial cells, polymerized to polyesters, and densely packed in granules. In the absence of extracellular carbon, the internally stored PHAs are depolymerized and consequently metabolized to enable cell maintenance and reproduction. However, some bacteria exhibit growth associated production and degradation of PHAs as part of the cell sustainment. This natural production and degradation cycle indicates that PHAs possess biodegradability and may have biocompatibility properties. Since the discovery that some bacteria can incorporate 3-hydroxyalkanoates bearing functional groups from related substrates, research has led to structural diversification of PHAs by biosynthesis and chemical modifications. A commonly applied route for tailoring PHAs is their in situ functionalization by biosynthetically producing side chains with terminal double bonds followed by chemistry. Non-functionalized PHAs can also be activated by surface modification techniques. The resulting tailor-made structural and material properties have positioned polyhydroxyalkanoates well to contribute to the manufacturing of second and third generation biomaterials.