There has been continued debate over the relative importance of crack closure and other sources of K max dependence in metallic fatigue crack growth. Crack closure is believed to be a very important physical mechanism that is a source for the observed dependence of fatigue on load ratio or K max . It is unclear whether the difficulty in observing some hypothesized crack closure is the result of the very small distances over which contact may occur, or whether new mechanisms which operate in front of the crack tip are affecting crack growth. From the existing experimental fatigue data, two driving force models have been developed, one based on crack closure and a β¬K effective term, and a second based on two terms: β¬K and K max . We show how these two driving force models are mathematically compatible with each other, examine what these models say about microstructural mechanisms of fatigue crack growth, and discuss the implications of each model on mechanical design of structures, materials design, and materials science. If there is to be continued debate over the relative importance of closure behind the crack tip and of the effects of stresses in front of the crack tip, we believe the debate should focus on the physical micro-mechanisms involved, rather than on analysis of the mechanical driving forces of crack growth or far field estimates of crack closure.