This chapter discusses steroid metabolism that concern either recent developments in analytic techniques or interpretations of complex endocrine laboratory data. It is frequently pertinent to determine accurately the production rate (PR) of a steroid hormone. “Production rate” is defined as the sum of the glandular secretion of a particular hormone plus all other compounds that contribute to this hormone or to the metabolites measured. In the past, the PR has been approximated or inferred from the measurements of urinary metabolite levels or blood levels. The urinary metabolite levels only approximate actual glandular production because of unpredictable alterations in metabolism, difficulties in hydrolysis of urinary steroid conjugates, and the fact that several precursors can yield the same metabolites. In an attempt to relate steroid blood levels to production rate, variations are produced by circadian rhythms and alterations in metabolic clearance rate. Thus, attention has shifted to the more direct measurement of steroid production rates using isotopic methods.