Rapid weight loss (RWL) means the manipulation of body mass in the last several days preceding a competition through different methods of training, and modifications in dietary and fluid intakes. Many studies describe the prevalence of methods of RWL, but there has been little empirical research into whether the methods athletes employ for RWL are effective. My literature review examined the prevalence, magnitude and methods of RWL in sports with weight classes, but with specific interest in combats sports and MMA. The review also explored mechanisms by how the various approaches produced RWL and how these approaches affect health and performance. Study 1 was a survey study employing the previously-validated Rapid Weight Loss Questionnaire to investigate current trends of RWL in Irish Mixed Martial Arts athletes. A novel finding was that hot (salt) baths were implemented by a high percentage of respondents, despite this method of RWL not being previously studied for its effectiveness in the weight cutting process. Study 2 investigated body mass losses during RWL using a hot bath protocol with salt in the form of Epsom salt (1.6%wt/vol) or without salt. In a crossover design, MMA athletes performed a 20-min immersion at a fixed temperature of 37.8°C followed by a 40-min wrap. This bath and wrap was performed twice per visit. The body mass loss in salt water baths (SWB) was similar to fresh water baths (FWB). Study 3 used the same protocol to investigate body mass losses during hot water immersion with or without salt, but this time with the temperature commencing at 37.8°C and self-adjusted by participants to their maximum tolerable temperature. Again the body mass lost in SWB was similar to FWB. Study 4 investigated the effects of a higher salt concentration (5.0%wt/vol Epsom salt) using the hot bath protocols in Studies 2 and 3 on body mass, blood-based markers, and indices of performance. The magnitude of body mass lost in SWB was similar to FWB, and there was no difference between conditions on blood markers or in the performance tests. Future research should focus on how to optimise the hot bath process in order to aid RWL. This may include investigating the ideal temperature of water, salinity or osmolality of water, and the duration of bathing.