The role of surface roughness on the fatigue and fracture behavior of a toughened epoxy adhesive system was investigated experimentally. Fatigue studies covered both the fatigue threshold strain energy release rate, G(th), and fatigue crack growth rates, while the strain energy release rate for crack initiation, G(c)(i), and the steady-state value, G(c)(s), were measured under quasi-static loading. Mixed-mode fatigue results showed a significant dependency on surface roughness. G(th) increased with roughness, reached a plateau, and then decreased for very rough surfaces. This increase in G(th) was explained in terms of the increase in bonding and fracture surface area, crack growth retardation due to the microtopography of the substrate, and crack path deviation from the interface. The decrease in G(th) for very rough substrates was attributed to void formation and stress concentration at the tip of asperities. The effect of roughness on fatigue diminished as the applied strain energy release rate increased. This was a result of the crack path becoming more cohesive, moving away from the interface. Similarly, no effect of surface roughness was observed in the mode-I fatigue results and the mixed-mode fracture results, since the crack path in these cases was far enough from the interface.