The mechanisms that control the release of components of nuclear waste glasses into aqueous environments and the rates of such release depend to a large extent on the contact time between the glass and a particular volume of water. At short contact times the release of leach products does not significantly affect the reactivity of the water toward the glass, while at long contact times, such as those expected in repository environments, saturation of the aqueous medium and the formation of new solid phases are very important. The development of a methodology based on the analysis of flow test data to identify controlling leach mechanisms under slow-flow as well as rapid-flow conditions is described. Not only are the leach mechanisms and leach rates strongly dependent on contact time, but the effects of glass composition, leachant composition, and temperature on the leaching process are as well. Accordingly, test data and models obtained for long contact times are most useful for developing predictions of glass durability under repository conditions, while the applicability of short-term test data is limited.