Improved muscle strength and aerobic fitness are of great importance in many sports and are also associated with increased life expectancy and a reduced incidence of a number of diseases. In this chapter, we will discuss some of the factors that influence muscle strength and aerobic fitness, including sex-related factors. The way physical fitness is measured is also important when making comparisons. An increased understanding of the assessment of strength and aerobic fitness may inspire fruitful improvements in practical test and training programs in various sport and health contexts.