A study of the electron dynamics in the dissipation region of collisionless magnetic reconnection is presented. The investigation is based on a new 2.5-dimensional electromagnetic particle-in-cell simulation code. This code is applied to the problem of reconnection in two differently sized current sheets: one with a thickness of the ion inertial length and the other with electron inertial length thickness. The complete set of contributions to the reconnection electric field is calculated directly from the particle information. The two cases lead to quite different results. In the ion scale, sheet reconnection is significantly slower, and the dissipation is provided virtually exclusively by electron quasi-viscous effects. The electron scale sheet reconnects much faster, involving a bifurcation of the reconnection region and the formation of a magnetic island. In this latter case, dissipation appears to be primarily provided by electron inertial effects and here foremost by bulk electron acceleration. Finally, an attempt to represent the effects of electron pressure-based dissipation in a transport model is presented also. ¿ 1998 American Geophysical Union