Fatigue (slow) crack growth in epoxy/glass, epoxy acrylate/glass and epoxy/PMMA interfaces was studied under constant and cyclic loading at both high and low humidities using the interfacial, four-point flexure test. Finite element analysis was used to determine the energy release rate and phase angle appropriate for the different crack geometries observed. The experimental results show that for the polymer/glass interfaces, the primary driving force for fatigue crack growth is the applied energy release rate at the crack tip and that increasing test humidity enhances crack growth under constant loading but has an insignificant effect under cyclic loading. At low humidity the crack growth rates under cyclic loading are significantly greater than under constant loading. For epoxy/PMMA interfaces the crack growth results were independent of the applied energy release rate, relative humidity, and cyclic rs. constant loading, within experimental scatter. In addition, for polymer/glass interfaces the effect of phase angle (13 to 54 degrees) on crack growth rates is not significant. However, for epoxy/PMMA interfaces the applied energy release rate for the initiation of crack growth is considerably greater for a phase angle of 66 degrees than for 5 degrees, indicating that increasing shear at the crack tip makes the initiation of crack growth more difficult. These results are discussed in terms of possible mechanisms of fatigue crack growth al polymer adhesive interfaces.