The measurement of physical fitness in children and youth has long been a topic of interest to physical educators, exercise scientists, health agencies, and private organizations dealing with sport and fitness. In recent years the focus on problems in measuring fitness has become more intense due to a number of factors, in particular the need for adequate surveillance studies and the perceived lack of fitness education in the schools. The purpose of this paper is to examine the scientific evidence that supports the use of measurement techniques for fitness evaluation. Physical fitness is described as a scientific construct that is multifaceted. A rationale is presented for the development of an operational definition of fitness that provides a basis for the identification of components of fitness to be measured. The latest versions of national fitness tests are described and compared, and evidence of the reliability and validity of the test batteries as well as individual subtests is summarized. Finally, several issues associated with fitness testing are discussed. These include the use of criterion referenced standards, the development of norms, and the use of fitness tests in a school setting. Although many improvements in tests and testing practices are noted, research is needed on a variety of problems.