The objective of this book is to inform all persons involved in the equine breeding and sports industry about the practical use of measurements of biochemical and physiological variables for the determination of athletic potential of horses. The effects of exercise and the effects of training are differentiated. The text discusses how changes induced by exercise can only be of practical importance for performance diagnosis when measurements are taken regularly in the same horse throughout its development and following repeated exercise, such as in training. However, the key to benefiting from any variable is standardization of the conditions under which samples and measurements are taken. This booklet aims to contribute to understanding which variables are important for performance diagnosis and how sampling and measurement should be done to obtain optimal results and thus encourage their use in practice.