PHAs (polyhydroxyalkanoates = biopolyester) composed of hydroxy fatty acids represent a rather complex class of storage polymers synthesized by various bacteria and archaea and are deposited as water-insoluble cytoplasmic nano-sized inclusions. These spherical particles are composed of a polyester core surrounded by phospholipids and proteins. The key enzymes of polyester biosynthesis and polyester particle formation are the polyester synthases, which catalyze the formation of polyesters. Various metabolic routes have been identified and established in bacteria to provide substrate for polyester synthases. Although not essential for particle formation, non-covalently attached proteins, the so-called phasins, can be found at the particle surface and are considered as structural proteins. Protein engineering of polyester synthases and phasins was used to shed light into the topology of these granule attached proteins. Biopolyesters and the respective micro-/nano-structures are currently considered as biocompatible and biodegradable biomaterials with numerous potential applications particularly in the medical field.