Photoelectron angular distributions of atomic and molecular systems are discussed. The theory is developed in terms of the single center expansion model, and the photoionization differential cross section is separated into terms corresponding to the various partial wave components of the photoelectron wavefunction. The differential cross section is decomposed into a series of (1) direct (incoherent) terms for each final state partial wave, plus (2) a series of interference (coherent) terms between the various partial waves. The resulting description is particularly useful for emphasizing the contribution of individual partial wave components to the differential cross section. Illustrative results are presented for the photoionization of the hydrogen molecule–ion. These results provide further evidence of the reliability of the noniterative integral equation method for studying photoionization processes.