This study aims to identify the fatigue crack growth mechanisms in a strong and tough Ni 78Si 10B 12 metallic glass and to examine the effect of fine crystalline particles on the crack growth behaviour. In common with most crystalline materials, the mechanism and rate of crack growth are dependent on the applied ΔK. In the amorphous alloy fatigue cracks can continue to grow at extremely low ΔK levels and only become dormant at 0.5 M Pa m 1 1 2 ( ΔK TH . Near threshold, the crack tip plastic zones consist of a single shear band extending ahead of the crack tip. At intermediate growth rates the plastic zone consists of a large number of shear bands similar in shape to the Prandtl slip line field for non-hardening materials. Decohesion along these bands produces undulating fracture morphologies. At high growth rates amorphous specimens develop shear lips under the control of K max . Isothermal heat treatment of the alloy at 440°C for 7 min leads to an improvement in the value of ΔK TH and reduces low crack growth rates without changing the basic micromechanisms of crack growth. This is due to homogenization of the slip distribution by the fine crystalline particles and to enrichment of the amorphous matrix by boron.