Polyesters consisting of hydroxyalkanoic acids (PHA) represent in addition to proteins, nucleic acids, polysaccharides, lignin, polyphosphates and polyisoprenoides an additional class of biosynthetic polymers that are synthesized in large quantities by living matter. PHA are like natural rubbers and their derivatives naturally occurring thermoplastics and elastomers and are currently produced biotechnologically with bacteria and in the future with genetically engineered plants. Occurrence, biosynthesis and bio-technological production as well as structures and properties of PHA are summarized.