We have calculated the piezoelectric field and charge distribution for various III-nitride heterostructures. Our calculations include strain energy minimization and doping effects, and are presented to show the magnitude of piezoelectric effects in strained layers. We compare our calculated results to device results where available. These include the two-dimensional electron gas in heterojunction field effect transistors, Schottky diodes with strained layers for Schottky height engineering, and III-nitride single quantum wells. Calculations that included energy considerations resulted in good agreement between predicted and observed field and charge distributions for the heterojunction fields-effect transistors structure.