Stem cells are the progenitors of specialized cell types in the body and have the ability to self-renew. They play critical roles during embryonic development and in the maintenance and repair of adult tissues. Given their plasticity and regenerative abilities, stem cells provide opportunities for treating human diseases such as diabetes. Written and edited by experts in the field, this collection from Cold Spring Harbor Perspectives in Biology examines recent progress in our understanding of stem cell biology and how the properties of stem cells can be manipulated for therapeutic purposes, emphasizing the roles that cutting-edge technologies and interdisciplinary approaches have played in this work. The contributors describe how the first cell lineages arise in the mammalian embryo, stem cell dynamics during the development and homeostasis of specific tissues (e.g., epithelia and brain), and what happens when stem cell integrity is compromised (e.g., by DNA mutations). They also explore specialized niches in which stem cells reside (e.g., in the testis and lung) and how the extracellular cues in those microenvironments regulate stem cell behavior. Many chapters illustrate how single-cell profiling, stem-cell-derived organoids, intravital microscopy, lineage tracing, quantitative modeling, and other modern approaches have offered important insights. In addition, the authors consider the potential of stem-cell-based therapies in the clinic (e.g., for treating retinal diseases and skin disorders) and the innovations that are facilitating the development of those therapies, including various lineage reprogramming strategies and new biomaterials that modulate stem cell properties. This volume is therefore an indispensable reference for molecular, cell, and developmental biologists, as well as anyone wishing to explore the possibilities of stem cells in regenerative medicine and tissue engineering.