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Inhomogeneous distribution of particles is unavoidable in the microstructuralscale of particulate reinforced metal matrix composites (PMMCs). This has aconsiderable influence on the short crack, and near threshold long crack growth.Generally particle clusters and often large particles are sites of short crack initiation.Once a short crack is initiated, its growth is highly influenced by the heterogeneousdistribution of particles. Two characteristics distinguish the short crack behavior inPMMCs. First, both the direction and growth rate are highly affected by particles,especially the large ones. Second, the size of short cracks which exhibit micro-structuresensitivityis much longer than that of the matrix material. In the case of long cracks thegrowth rate in the intermediate Kmax (or ÄK) is independent of particles. However,particles increase resistance to crack growth at the low (near threshold) values. Basedon the observed behavior of short and long cracks in PMMCs, six crack growth regimeshave been identified. Each phase boundary corresponds or is related to an overallmaterial property.