A two dimensional, time dependent model has been developed for the prediction of droplet formation in Gas Metal Arc Welding. The model is a unified treatment of the arc, the anode welding wire and the cathode work piece. Predictions are made of the formation and shape of the welding droplets as a function of time, accounting for effects of surface tension, gravity, inertia, plasma pressures, viscous drag and magnetic pinch forces, and also for the effects of wire feed rate and gas flow rates. For an arc in argon with a mild steel wire of 1.6 mm diameter and a current of 300 A or more, we predict the formation of small drops of diameter 1.2 mm or less and high drop frequencies, consistent with the spray transfer mode observed in welding. At currents of less than 275 A, we predict large drop sizes of more than 3.9 mm in diameter, consistent with the globular transfer mode in welding.