The ability to measure accurately the hormones regulating calcium homeosta­ sis is the fundamental first step toward understanding the roles these hormones play in health and disease. Techniques for such measurements have only been available for the past 10 years or so and remain in a state of rapid development. Sensitive parathyroid hormone (PTH) radioimmunoassays appeared in the early 1970s, and with them came a whole new appreciation for the prevalence and implications of hyperparathyroidism, primary or secondary, in the popu­ lation. The calcitonin (CT) radioimmunoassay came later and achieved rapid success in the. diagnosis of a previously poorly understood cancer, medullary carcinoma of the thyroid, frequently associated with the familial multiple endo­ crine neoplasia type 2 syndromes (a and b). As the sensitivity of the calcitonin radioimmunoassay has improved, our understanding of the role of calcitonin in normal physiological processes has increased. The knowledge that vitamin D must be metabolized to produce its biologic effects is only 15 years old. This has had profound implications in our understanding of a variety of metabolic bone, kidney, and gastrointestinal diseases. Assays to measure the major cir­ culating form of vitamin D, 25-hydroxyvitamin D, were described 10 years ago. Assays for the other metabolites, in particular, 1,25-dihydroxyvitamin D, were described even more recently. As of today, we know of many vitamin D metabolites and have developed the techniques to measure most of them; how­ ever, many questions remain concerning their physiological role.