‘Heat cramping’ is defined here as severe, spreading, sustained, sharply painful muscle contractions that can sideline athletes. Not all cramps are alike, but three lines of evidence suggest heat cramping is caused by ‘salty sweating’, specifically by the triad of salt loss, fluid loss and muscle fatigue. The first line of evidence is historical. Dating back 100 years, heat cramping in industrial workers was alleviated by saline, and in a self-experiment, salt depletion provoked muscle cramping. The second line of evidence is from field studies of athletes. In tennis and football alike, heat-crampers tend to be salty sweaters. Some evidence also suggests that triathletes who cramp may lose more salt during the race than peers who do not cramp. The third line of evidence is practical experience with therapy and prevention. Intravenous saline can reverse heat cramping, and more salt in the diet and in sports drinks can help prevent heat cramping. For heat cramping, the solution is saline.