Although only a subfield of human toxicology, human stem cell toxicology faces greater challenges. Beyond well studied problems in human toxicology like determination of human exposures, causative agents, and toxicity mechanisms, the development of effective human stem cell toxicology concepts and practice will require solutions for two largely unyielding problems that impede progress in all research and applications involving postnatal tissue stem cells. These are (1) identifying tissue stem cells with sufficient specificity to count them; and (2) isolating or producing tissue stem cells in sufficient purity and number for specific assay development. The inherent and unique properties of postnatal tissue stem cells conspire to present these long-standing challenges. Tissue stem cells exist at minute fractions in tissues, and their unique property of asymmetric self-renewal keeps their fraction low while they continuously produce lineage-committed cells that confound their detection, both morphologically and molecularly. The crucial role of stem cells in fetal and postnatal tissue function, health, aging, and disease makes understanding their toxicology an imperative for environmental health science and medicine. Chapter 1 provides an overview of the topics presented in Human Stem Cell Toxicology that begin to illuminate new paths to improved human stem cell toxicology concepts and practice.