Acoustic emission monitoring has been successfully employed in different application areas such as monitoring of fatigue loaded bridges and pressure testing of vessels. However, application of this technology on resistance (spot) welding processes yielded limited results. Existing literature only mentions the detection of certain defects such as expulsion and cracked welds. Detecting nugget nucleation and growth, to allow prediction of nugget diameter, has never been proven to be feasible based on acoustic emission signals. This work contains experiments based on varying as well as constant nominal welding parameter sets, where acoustic emission waveforms were collected during the welding process. A methodology is described, revealing a correlation between certain frequencies present in the acoustic emission signal and physical events occurring during welding. Based on these frequencies, a model is constructed for predicting the nugget diameter of resistance spot welds based on acoustic emission measurement data.