Polyhydroxyalkanoates (PHAs) are synthesized by numerous bacteria as carbon and energy storage compounds, and are good candidates for biodegradable plastic material. With the current advances in PHA research, PHA concentrations of greater than 80 g l(-1) with productivities of greater than 2 g PHA l(-1) h(-1) can be routinely obtained by fed-batch cultivation of several bacteria. Metabolic engineering approaches have been used to expand the spectrum of utilizable substrates and to improve PHA production. These advances will lower the price of PHA from the current market price of ca. US$16 kg(-1), and will allow PHA to become a leading biodegradable plastic material in the near future.