An overview of leak testing methods for heat exchangers is presented. The problems associated with the presence of residual liquid during testing were assessed as it was shown experimentally that complete drainage of plate type heat exchangers is very difficult. A theoretical modelling of small leaks in heat exchanger plates was combined with laboratory experiments. The strengths and weaknesses of electrolytic differential analysis and helium leak detection methods are presented. The electrolytic differential analysis method is capable of detecting smaller leak volumes than the helium leak detection method, in the presence of water, assuming α 0.5 μS/cm and 0.1 ppm helium detection limit for the former and latter cases respectively. The test pressure was shown to be a significant factor in the detection of very small holes; this was a particularly important criterion for gas based hole detection methods in the presence of water.