Retinoic acid (RA) is a signaling molecule involved in many important pathways in every cell. When RA binds to RA receptors and activates them, cell cycle arrest, differentiation and apoptosis can occur. In this book, the authors present topical research in the study of the structure, mechanisms and roles in disease of retinoic acid. Topics discussed include retinoid signaling in mammalian inner ear development; nuclear receptors for retinoids involved in the control of expression activity of p-glycoprotein; retinoic acid signaling and immunotherapy of cancer; using retinoic acid in molecular medicine and retinol/vitamin A signaling and self-renewal of stem cells.