In this paper we challenge the classical explanation of Mach bands--that they result from lateral inhibitory mechanism operating in the visual system--and present an alternative explanation based on a recent local energy model of feature detection (Morrone & Burr, 1988). A series of experiments was conducted to establish the range of parameters under which Mach bands appear on periodic waveforms of the trapezoid family. The model predicts successfully the conditions under which Mach bands appear, and the contrast necessary to see them. Other models, including those based on lateral inhibition and band-pass filtering fail to do so.