A theory of fast collisionless reconnection is presented. Because of the Hall term in Ohm's law, electron and ion dynamics decouple on scales smaller than the ion inertial length $\frac{c}{{\ensuremath{\omega}}_{\mathrm{pi}}}$. Though reconnection requires electron inertia to break the frozen-flux constraint, the rate is essentially independent of the electron mass. Instead, ion inertia controls the reconnection rate, which is strongly enhanced compared to previous estimates in which Hall effects were neglected.