This paper seeks to show that a possible explanation of excessive exchange rate variability is that the dynamics of exchange rates are being driven by nonlinearities in the underlying economic relationships. The Dornbusch model, which is the theoretical underpinning of many models of exchange rate dynamics, is the starting point. A nonlinear demand function for foreign assets is introduced into the model. Small time lags, in the adjustment of exchange rates to the differential between domestic and foreign interest rates, and, in the formation of expectations of exchange rate changes are both introduced. The dynamics of the model are governed by a set of three nonlinear differential equations. The system is analysed using concepts and techniques from the theory of noninear differential equations. The model exhibits various dynamic regimes as the expectations lag decreases, moving from global asymptotic stability (long lag), through limit cycle behaviour to a discontinuous oscillation (short lag). The behaviour for short time lags, and in the perfect foresight limit, is consistent with highly variable exchange rates.