Application of polyhydroxyalkanoates (PHAs) and related biodegradable polymers has gained momentum in various areas of biotechnology. A promising application that started appearing in the past decade is the use of PHAs as the solid substrate for denitrification of water and wastewater. This type of denittification, termed here "solid-phase denitrification", has several advantages over the conventional system supplemented with liquid organic substrate. PHAs serve not only as constant sources of reducing power for denitrification but also as solid matrices favorable for development of microbial films. In addition, in contrast to conventional processes, the use of PHAs has no potential risk of release of dissolved organic carbon with the resultant deterioration of effluent water quality. If the production cost of PHAs can be brought down, its application to the denitrification process will become economically more promising. A number of PHA-degrading denitrifying bacteria have been isolated and characterized from activated sludge and continuous flow-bed reactors for denitrification with PHAs. Most of these isolates have been assigned phylogenetically to members of beta-Proteobacteria, especially those of the family Comamonadaceae. The metabolic and regulatory relationships between PHA degradation and denitrification, and the interactive relationship between PHA-degrading cells and the solid surface structure are important subjects awaiting future studies, which would provide a new insight into our comprehensive understanding of the solid-phase denitrification process.