Steroids regulate differentiation, growth and functioning of many tissues via a nuclear receptor gene activation mechanism. Molecular biology has opened the way for more detailed investigation of the complex set of intracellular interactions contributing to organ-selective actions of steroids and steroid-like compounds such as vitamin D. In this volume leading experts address basic molecular mechanisms of tissue specificity and related issue, with the goal of attaining a better understanding of the underlying mechanisms of organ selective actions of steroids. Discussed is the extent to which tissue specificity might selectively be modified by chemical modification of the hormones and how this may be applied towards improved therapies by taking advantage of tissue selectivity.