Body size, body composition, and physiology are basically similar in boys and girls before puberty. The adolescent growth spurt and puberty mark the period in life when sex differentiation and development begin. Peak height velocity for girls ranges from 10.5 to 13 years and for boys, 12.5 to 15 years. Peak weight velocity and menarche occur approximately 6 months and 1 year, respectively, after the height peak. On average, women have a larger surface area-to-mass ratio; lower bone mass; and wider, shallower pelvis compared with men. The implication of these characteristics is that women may have an advantage in dry heat, are more at risk for osteoporosis, and may be predisposed to experiencing knee problems. Overall, adult women have more body fat, less lean muscle mass, a gynoidal fat distribution, a lower resting metabolic rate, lower red blood cell mass, and lower hemoglobin and hematocrit compared with adult men. The difference in muscle strength between trained women and men can be explained by muscle mass size and not to differences in muscle fiber type or muscle adaptation. The lower resting metabolic rate is not related to gender per se, but to the fact that muscle mass is more metabolically active compared with fat. The lower VO2max in female athletes is explained primarily by differences in body composition and oxygen transport system between the sexes. A major problem occurring among female athletes is the misuse of prescribing low body fat for appearance and performance. The outcome is an athlete who appears to be in a negative caloric balance, and who is deficient in key nutrients. The consequences could result in the following cascading problems: disordered eating, iron deficiency anemia, amenorrhea, premature osteoporosis, and injuries.