There is a growing body of research in economics that studies the consequences of time-inconsistent preferences. This paper introduces time-inconsistent preferences in a general equilibrium setting. We discuss how the standard notion of competitive equilibrium should be extended in order to allow for changes in intertemporal preferences. Depending on whether or not agents recognize that their intertemporal preferences change, agents are called sophisticated or naive. We present competitive equilibrium notions for economies with naive agents and economies with sophisticated agents and provide assumptions under which both types of equilibria exist. Time-inconsistency also raises conceptual issues on the appropriate notion of efficiency. Of particular importance is the way future intertemporal preferences are taken into account. An example shows that sophisticated equilibria may be less efficient than naive equilibria. We specify suitable conditions for which both types of equilibria satisfy appropriate efficiency notions.