When it comes to sports nutrition, athletes often prioritise their macronutrient intake, viewing hydration as secondary, and only focusing on their fluid and electrolyte needs if issues start arising. In this chapter, we discuss the evolutionary basis for why exercising humans sweat so much, how this has the potential to lead to levels of dehydration that can significantly impact an athlete’s performance, and therefore why hydration should be considered a priority in the planning for training and competition. Hydration can be a confusing topic for athletes, with many conflicting views and evidence published on what, when, and how much athletes should be drinking. To help athletes and coaches navigate this ‘debate’, this chapter reviews both historical and recent research and advice on fluid and electrolyte use for hydration. Recommendations for hydrating before, during and after exercise bouts are provided, taking individual factors and exercise outcomes/ goals into consideration. Case studies and practical tips are included to help athletes and practitioners understand the individuality of sweat losses and personalisation of hydration strategies, along with different methods to do so. The potential role of dehydration and electrolyte imbalance in the development of exercise associated muscle cramps is also discussed.