Enhanced control of the short-circuit gas metal arc welding (GMAW) process can be achieved through the use of an advanced power source applying a controlled current waveform. By using a current waveform whose period is adapted to suit the requirements of the process, excellent welding performance can be achieved. When the current waveform is of fixed peak amplitude and shape, the control can be considered to be open-loop, as there is no active attempt to regulate a specific process parameter such as arc length, fusion area or dipping frequency. There is scope for more sophisticated control of the process. Using a typical open loop control strategy as a benchmark, an alternative closed loop control is evaluated here. The technique regulates the size of the droplet formed on the end of the electrode on a cycle-by-cycle basis. The instantaneous melting rate is modelled in a real-time welding controller, and the current waveform is adapted to achieve the desired droplet size, as determined from the time integral of the instantaneous melting rate.