Peening is a technique commonly used to improve fatigue resistance, but it is not always appreciated that excessive peening may be detrimental to fatigue behaviour. The present work demonstrates this effect in three different alloys: a mild steel, stainless steel, and a commercial copper alloy. The dependence of fatigue behaviour on peening intensity is shown to reach a maximum beyond which high intensity peening reduces fatigue life; at very high intensities the fatigue life can be reduced below the unpeened value. This reduction in fatigue life is shown to coincide with an increase in surface roughness, as measured by the parameter Ra, and the onset of a distinct damage mechanism revealed by scanning electron microscopy. The varying effect of peening intensity in the different materials is related to their tensile, fatigue, and wear properties; a model is advanced to explain the observed behaviour, based on the change from an initiation dominated mechanism to a propagation dominated mechanism as peening intensity is increased. A limited number of results is also presented showing the effect of peening time on fatigue behaviour.