Microcracks in bone acts as a stimulus for bone remodelling, contributes to the loss of bone quality in osteoporosis and the formation of fragility and stress fractures. This study developed a novel method of crack detection to label microcrack initiation, growth and coalescence during bone fatigue. The results show that cracks accumulate early in a bone''s life but the rate of growth slows until before a second increase in accumulation just prior to failure. The majority of microcracks are found in interstitial bone between osteons with crack growth occurring parallel to the longitudinal axis of the bone resulting in an elliptical shape with a 5:1 longitudinal: transverse ratio. This supports the concept of a microstructural barrier effect whereby osteons act as barriers to crack propagation in compact bone. However, it shows that this microstructural barrier effect is dependent on crack length. For the vast majority of cracks, osteons act as barriers to growth but for the minority of cracks that are long enough and do break through the cement line, an osteon may actually act as a weakness in the bone and facilitate crack propagation.