Two models of fatigue failure (designated type 1 and type 2) have been investigated by use of a total of seven materials. For type 1 behavior, there is a clear relationship between fatigue life and fatigue stress, whereas for type 2 behavior no such relationship exists. For materials exhibiting type 2 behavior, failure occurs at a level of stress below the ultimate strength of the material, but the values of fatigue life appear to be distributed randomly when several specimens of the same material are tested. A value of fatigue limit can be calculated for these materials which is independent of fatigue life. It is likely that, for these materials, resistance to fatigue is primarily dependent upon the presence or absence of flaws such as porosities. For the dental materials investigated in this study, it appears that brittle materials such as dental plaster and heavily filled composites are likely to exhibit type 2 behavior, whereas less brittle materials, such as more lightly filled composites, are more likely to exhibit type 1 behavior.