Fulfilling individual energy and nutrient requirements is of great importance for athletes to support overall health and well-being, training adaptation, recovery and injury prevention. Energy availability is the amount of energy left over and available for bodily functions after the energy expended for training is subtracted from the energy taken in from food. The syndrome of Relative Energy Deficiency in Sport (RED-s) refers to the multifactorial health and performance consequences of low energy availability. Potential physiological implications of RED-s include impaired metabolic rate, hormonal disruptions, menstrual dysfunction, reduced bone health, immunity, protein synthesis, and cardiovascular health. These can have short and long term consequences on health and sport performance. Causes of RED-s range from unintentional (e.g. lack of awareness or difficulties with meeting high energy requirements) to more intentional behaviors and further to clinical eating disorders. RED-s prevalence appears to differ between sports and sport disciplines, with highest risk in endurance, aesthetic and weight-class sports. This article summarizes current knowledge of RED-s implications for health and performance, and highlights the importance of early diagnosis and screening. Research on RED-s in Icelandic athletes is warranted as it could support development of national guidelines, prevention and treatment protocols.