This paper describes crack growth processes in carbon fibre and glass fibre-epoxy laminates under stress and quantifies by physical modelling the accumulation of damage with load cycling. Cracking mechanisms are observed directly and identified by in-situ dynamic scanning electron microscopy. Armed with this information, particular attention is given to the differences between these cracking processes and the way they interact, their growth-rate, and their effect on the damage-state of the structure with variations in laminate geometry, with changes in magnitude of applied stress, and with fatigue cycles. Particular attention is given to the internal damage state variable approach to physical modelling.