The topic of plasticity-induced closure and its role in shielding a crack tip from the full range of applied stress intensity factor has provoked considerable controversy over several decades. We are now in an era when full field measurement techniques, e.g. thermoelasticity and photoelasticity, offer a means of directly obtaining the stress field around a crack tip and hence the effective stress intensity factor. Nonetheless, without a clear understanding of the manner in which the development of plasticity around a growing crack affects the applied stress field, it will remain difficult to make crack growth rate predictions except through the use of an often highly conservative upper bound growth rate curve where closure is absent, or through semi-empirical approaches. This paper presents new evidence for an interpretation of plasticity-induced crack tip shielding as arising from two separate effects; a compatibility-induced interfacial shear stress at the elastic-plastic interface along the plastic wake of the crack, and a crack surface contact stress which will vary considerably as a function of stress state, load and material properties.