A review of fatigue crack propagation and near-threshold surfaces in alpha / beta titanium and iron-base alloys reveals at least 10 types of alternate fracture processes, compared with the more common striation process. Microstructural influence was found in all materials with interactive effects of temperature, frequency, and environment. At low temperatures, alternate cleavage modes produce crack growth exponents that increase in a consistent way with either a decrease in test temperature or a decrease in fracture toughness. Such phenomena may be understood in terms of void-induced fracture strain concepts or, alternatively, with strain-rate sensitivity concepts applied to ductile ligaments. Factors of two to three increase in threshold are shown to exist through microstructural modification of either Widmanstaetten colony or grain size. A semicohesive ligament zone based upon a modified Dugdale-Barenblatt model is shown to predict both of these microstructural effects. It is emphasized that such models are in their formative stages and require additional modification to include overload, R-ratio, and strain-hardening concepts.