Hydrolysis of phosphate esters is a critical process in the energy metabolism and metabolic regulation of plant cells. This review summarizes the characteristics and putative roles of plant acid phosphatase (APase). Although immunologically closely related, plant APases display remarkable heterogeneity with regards to their kinetic and molecular properties, and subcellular location. The secreted APases of roots and cell cultures are relatively non-specific enzymes that appear to be important in the hydrolysis and mobilization of Pi from extracellular phosphomonoesters for plant nutrition. Intracellular APases are undoubtedly involved in the routine utilization of Pi reserves or other Pi-containing compounds. A special class of intracellular APase exists that demonstrate a clear-cut (but generally nonabsolute) substrate selectivity. These APases are hypothesized to have distinct metabolic functions and include: phytase, phosphoglycolate phosphatase, 3-phosphoglycerate phosphatase, phosphoenolpyruvate phosphatase, and phosphotyrosyl-protein phosphatase. APase expression is regulated by a variety of developmental and environmental factors. Pi starvation induces de novo synthesis of extra- and intracellular APases in cell cultures as well as in whole plants. Recommendations are made to achieve uniformity in the analyses of the different APase isoforms normally encountered within and between different plant tissues.