Book excerpt: Nuclear receptors are ligand activated transcription factors that control numerous biological functions. Consequently, altering activity of these receptors is proposed, and indeed documented, to affect many physiological and pathological conditions in experimental animals and humans. Thus, nuclear receptors have become a major target in the effort to treat numerous diseases. This book will shed light on and emphasize intricate processes involved in designing as well as discovering physiological and pharmacological modulators of these important proteins. World-renowned scientists will share with the reader their professional expertise and extensive experience acquired through decades working with nuclear receptors. Chapters address the various means and consequences of modulating nuclear receptor activity will be presented and discussed. These modulators cover a wide span of moieties ranging from synthetic chemicals to natural products. In addition, the classification of these chemicals ranges from pan agonists to selective agonists and inverse agonists to antagonists. They also include proteolytic means to obliterate the receptor in the event that modulating its activity through canonical pharmacological agents becomes less effective and/or less desirable due to anticipated or experienced toxicities. Modulation of receptor activity may also take place in the absence of a ligand or through manipulating the structure of the receptor itself by controlling posttranslational events.