The field of body-composition research currently lacks a systematic organization of methods used to quantitate components at the atomic, molecular, cellular, tissue-system, and whole-body levels of body composition. In this report we propose a classification system for body-composition methodology that proceeds in steps, beginning with division of methods into in vitro and in vivo categories, advances to organization by measurable quantity (property, component, or combined), and ends with grouping of methods by mathematical function (types I and II). Important characteristics of component-based methods are then developed, including a classification of component relationship types, the role of ratios and proportions in type II component-based methods, and the basis of simultaneous equations in multicomponent methods. This classification system, the first founded on a conceptual basis, explains similarities and differences between the many diverse methods, provides a framework for teaching body-composition methodology theories to students, and suggests future research opportunities.