Intuitively one would think that good flexibility (both extent and dynamic) would reduce the risk of injury as well as enhance performance, and the limited evidence available would tend to support this view. However, much of this evidence is based on studies of young fit sportspersons; we cannot conclude that the same principles apply to the general population until well-designed studies, using suitable instruments, have been performed. Reliable tools for measurement of flexibility are described, together with statistical methods considered appropriate for quantifying reliability. Recently described methods present the opportunity to investigate further, and it seems likely that dynamic flexibility will be a useful parameter to study. In all probability, prediction of fitness-for-task should be based not just on flexibility measures but on multivariable models; flexibility measurement is but one of a number of parameters to be viewed interactively in relation to the specific task or sport under consideration.