Successful cross-country skiing, one of the most demanding of endurance sports, involves considerable physiological challenges posed by the combined upper- and lower-body effort of varying intensity and duration, on hilly terrain, often at moderate altitude and in a cold environment. During the competitions lasting from 12 min (4 3-min sprint skiing races) to more than 2 h (for a 50-km race), cross-country skiers must employ a variety of techniques and change often between these, which is highly demanding, both physiologically and in terms of coordination. Over the years, research on this unique sport has also helped physiologists gain novel insights into the limits of human performance and regulatory capacity and there is a long-standing tradition of researchers in this field working close together with coaches and athletes to improve training routines, monitor progress, and refine skiing techniques. Integration of physiological and biomechanical approaches has and continues to contribute to more detailed and accurate analysis of several determinants of the performance of cross-country skiers, thereby promoting significant improvement.