There are some phenomena involving magnetic reconnection in collisionless laboratory and space plasmas that are characterized not only by fast growth but also by an impulsiveness, that is, a sudden increase in the time derivative of the growth rate. Three examples are the sawtooth collapse in tokamaks, the sudden enhancement of the cross-tail current density just prior to substorm onset in the Earth's magnetotail, and the impulsive phase of a solar flare. Not only is the peak growth rate of these phenomena large (near-Alfv¿nic), they are triggered suddenly, with the growth rate increasing from small to large values very rapidly. Traditional steady state reconnection models, such as those due to Sweet-Parker or Petschek, do not include time dependency, and hence cannot account for the impulsiveness. It is demonstrated that in collisionless plasmas, the effects of electron pressure gradients and Hall currents (referred to collectively as Hall MHD effects) can provide a plausible explanation not only of the fast growth but also of the impulsiveness. The geometrical structure of thin current sheets realized during the dynamical evolution of such collisionless systems changes spontaneously from a Y point in the early nonlinear regime (characteristic of the models of Sweet-Parker and Syrovatsky) to an X point (characteristic of the Petschek model). However, the underlying physics of reconnection is qualitatively different from that in resistive MHD models. In Hall MHD, the electron and ion dynamics are decoupled from each other in the reconnection layer. Such decoupling is forbidden in resistive MHD. In Hall MHD, while the spatial scale of the parallel electric field (that is, the component of the electric field parallel to the background magnetic field) is the ion skin depth, the spatial scale of the parallel current density is narrower, determined by the Lundquist number (or the electron skin depth), if resistivity (or electron inertia) is the dominant mechanism that breaks field lines. Analytical results as well as high-resolution numerical simulations are presented, and the predictions of the model are compared with data from laboratory experiments as well as space. ¿ 1999 American Geophysical Union