Predicting the service life of adhesive joints under fatigue loading remains a major challenge. A significant part of this task is to develop laws that govern the crack initiation phase. This paper contributes to this area through the development and application of the backface strain technique. A numerical study was carried out to investigate the effect of key parameters on the technique and to determine optimum gauge specification and location. Calibration curves were then produced relating the change in strain to the extent of damage. These numerical studies were then validated by undertaking a series of fatigue tests on both aluminium and GRP (glass-reinforced polymer)-bonded joints. Following various degrees of predicted damage the joints were carefully sectioned, polished, and studied using optical microscopy. The predicted and observed damage showed close correlation. The fatigue tests have also indicated that, for unmodified joints (intact fillets), even at high loads (50% static failure load) there was an initiation phase that accounted for about half the fatigue life of the joint. Removal of the adhesive fillet has been found to eliminate the initiation phase and consequently reduce fatigue life.