There are currently no calibration models that allow whole body density in professional footballers to be estimated. As such, there is a need to develop practical calibration models in order to make sound body composition judgements. The aim of this thesis is threefold. Firstly, to examine the measurement reliability of a range of anthropometric measures, residual lung volume, air displacement plethysmography and hydrostatic weighing. Secondly, to establish reliability and precision of body composition measures used within existing calibration models which estimate whole body density from the criterion of hydrostatic weighing. Thirdly, to develop and cross-validate new calibration models for professional footballers.