Physical fitness is a measure of the body's ability to perform physical activity and exercise, thus providing a valuable indicator of health (Tomkinson, Carver, et al., 2017). Cardiorespiratory endurance (CRE), also known as cardiovascular fitness, cardiorespiratory fitness or aerobic fitness, is an essential component for physical fitness and good health in general. Researchers have shown that physical conditions and fitness work during childhood and adolescence are determining factors for good health both current and future. Furthermore, there is evidence pointing to the long-term effect of adolescent physical activity on adult morbidity and mortality, along with growing evidence that physical inactivity during adolescence carries over into adulthood. However, recent research studies reported that many adolescents fail to hit the recommended levels of weekly physical activity, and they show, in addition, a worrying and progressive decline in the physical fitness levels of the current generation of adolescents when compared with teenagers of previous decades (Ferrari, Matsudo, & Fisberg, 2015; Suris, Michaud, Chossis, & Jeannin, 2006). Although numerous studies have analyzed the sports habits of the Spanish population (García- Ferrando & Llopis, 2011), there is a paucity of quantitative data on the evolution of fitness conditions, and specifically CRE evolution, in Spanish adolescents. The aim of the present study was to track the trends and changes in cardiorespiratory endurance among Spanish adolescents over a 20 year period (1998-2018).