Abstract: The development of nonperturbative quantum electrodynamics has made it possible to use exact wavefunctions and nonperturbative scattering theory to study multiphoton effects. Within this framework we develop a theoretical approach to interpret photoelectron angular distributions in terms of a generalized phase Bessel function which can determine the principal features of the distribution and its variation with laser frequency and intensity. Our theory has successfully explained a serious of experimental observations and predicted several new types of photoelectron angular distribution. A scaling law has also been established by this theory.