An investigation has been made into the propagation of fatigue cracks in Mode III (anti-plane shear) in circumferentially notched cylindrical specimens subjected to cyclic torsion with and without small superimposed static tension loading. Studies were performed in a range of low-alloy steels of yield strengths varying from 310 to 1625 MN/m², with the objective of characterizing both constant amplitude and variable amplitude Mode III crack growth behavior. Rates of fatigue crack propagation in Mode III were found to be characterized uniquely by the Mode III cyclic crack tip displacement (ΔCTD_III) or the plastic strain intensity range (ΔΓ_III) for elastic/plastic and fully plastic conditions, only if allowance was made for friction, rubbing, and interlocking between sliding crack surfaces (crack surface interference) through superposition of small axial loads. Such Mode III crack advance is envisaged in terms of Mode II shear coalescence of microcracks initiated at inclusions in the immediate vicinity of, and parallel to, the main crack front, consistent with micromechanical modelling studies incorporating either instantaneous crack tip displacement or damage accumulation arguments. The application of such models to the prediction of Mode III fatigue crack propagation behavior under both constant amplitude and simple variable amplitude loading spectra is briefly reviewed.