Knowledge on body composition is important both in health and in disease, especially when considering chronic (i.e., growing, ageing, pregnancy) and interventional (nutrition, exercise, physical training) biological processes, as well as in predicting, preventing and managing such modern chronic diseases as sarcopenia, obesity, type 2 diabetes or metabolic syndrome. Therefore, in this chapter we present human body composition by taking advantage of widely accepted models, from the simplest 2-compartment model, which allows fat and fat-free body mass to be distinguished, up to a more complex, 6-compartment model, capable of distinguishing body fat, total body water, bone minerals, proteins, soft-tissue minerals, and glycogen. Moreover, in this chapter we provide equations that can be applied in laboratory and clinical practice to predict the main components of body composition, such as body water, skeletal muscles (SMs), body fat, and bone minerals. Special attention is given to the methods of evaluation of SM mass and its importance in health and disease.