Assessment of the health status of animals through measurement of cellular, biochemical, and macromolecular constituents in blood, secretions, and excretions has been variously referred to as clinical chemistry, clinical biochemistry, or clinical pathology. The genesis of this discipline occurred in the mid-1800's, although the applications to medical and veterinary practices did not blossom until after the second world war when automated equipment required for processing large numbers of samples became available. Clinical biochemistry has now become a standard part of all diagnostic protocols for investigating health problems in humans and domestic or captive animals. A suitable group of tests can be defined to evaluate the function of most major organs, the endocrine system, the immune system, and the nervous system. By using this approach, the affected organs and, potentially, the processes responsible for the observed disease syndrome can be identified and further diagnostic tests called into play to make a precise diagnosis. While clinical biochemistry panels have been applied to free-ranging vertebrates only infrequently, the chapter will show that studies of captive animals indicate that such methods are now available.