AbstractTheoretical predictions using Finite Fracture Mechanics (FFM) normally treat the material as a homogeneous continuum. However, the use of FFM involves (or implies) a material length parameter, L, which may or may not be constant depending on the version of FFM used. In some cases this length parameter is the same order of magnitude as certain features in the microstructure (e.g. the grain size) but in other cases there is no obvious correlation to any microstructural distance. This paper will review our current knowledge on the relationship between L and microstructural parameters, for which there is data spanning several orders of magnitude. By using a new theoretical approach – the analysis of some thought experiments using simplified model microstructures, I show how the critical distance L can be related to a dominant microstructural distance d, depending on the operative mechanism of crack extension and toughening. This approach allows us to consider microstructure and micromechanisms in the context of FFM analysis.