Stimulated by the commercial availability of bacteriologically produced polyesters such as poly[(R)-3-hydroxybutyric acid], and encouraged by the discovery of new constituents of polyhydroxyalkanoic acids (PHA), a considerable body of knowledge on the metabolism of PHA in microorganisms has accumulated. The objective of this essay is to give an overview on the biodegradation of PHA. The following topics are discussed: (i) general considerations of PHA degradation, (ii) methods for identification and isolation of PHA-degrading microorganisms, (iii) characterization of PHA-degrading microorganisms, (iv) biochemical properties of PHA depolymerases, (v) mechanisms of PHA hydrolysis, (vi) regulation of PHA depolymerase synthesis, (vii) molecular biology of PHA depolymerases, (viii) influence of the physicochemical properties of PHA on its biodegradability, (ix) degradation of polyesters related to PHA, (x) biotechnological aspects of PHA and PHA depolymerases.