Although hype and hyperbole may sometimes seem synonymous with stem cell research, the fundamental technology offers very exciting opportunities for endocrinology research. In this issue of Endocrinology, you will find four minireviews, each demonstrating how this emerging technology may be applied to addressing questions in endocrine biology. Stem cells are defined as cells that undergo so-called asymmetric cell division, yielding both a self-renewing population of cells and giving rise to differentiated cells (1). The notion of stem cells, pluripotent cells, and multipotent cells emerged from the hematopoietic field, initially through histological characterization. These concepts are now widely accepted for most tissues, including those of interest to endocrinologists. In her review of pancreatic progenitor cells, Ku (2) emphasizes the concept, derived from the hematopoietic system, that the stem cells and the multipotent progenitor cells are minor populations with each subsequent level being more abundant. The actual identification of a tissue stem cell, be it in the breast (3), ovary (4), prostate (5), or pancreas (2), was originally through morphological criteria; in each minireview the authors emphasize the importance of more rigorous criteria for identification and characterization, including molecular markers and functional assays. Debates about stemness although arguably arcane, certainly serve to emphasize the difficulty of identifying stem cell populations and the need for rigorous criteria. The issues of reproductive cloning, therapeutic cloning, regenerative medicine, and the ethics of stem cell technology are of undeniable importance; they have, however, been extensively canvassed elsewhere (6, 7). In selecting this series of minireviews, we sought to highlight the potential that this technology has to enhance research in endocrinology. The difficulty of treating estrogen receptor-negative breast cancer and the relapses observed in the face of apparently effective endocrine ablation therapies provide a major impetus for understanding growth and development in the mammary epithelium. LaMarca and Rosen (3) outline their work and that of others, which has important insights into the development of the mammary epithelium. The notion that cancer may be a disease of stem cells has achieved considerable currency, making the search for a breast stem cell particularly pertinent (3). Although not the central focus of their work, LaMarca and Rosen (3) describe work in which stem cells/pluripotent cells have been used to develop a mammary ductal tree; such a system may provide ex vivo models for the study of this complex epithelium with the facility of an in vitro system, which allows an array of analyses and retains the intrinsic complexity of the tissue. A major driver in the study of pancreatic progenitors is the possibilities associated with regeneration of pancreatic islets/ -cells and the consequent implications for diabetes treatment. The full molecular characterization of the pancreatic developmental program also provides unique insights and a critical platform for studies of the biology of islet cell function (2). The reviews of both Risbridger and Taylor (5) on prostatic stem cells and Tilly and Rueda (4) emphasize the importance of stromal interactions, although from a very different perspective. Tilly and Rueda (4) emphasized the ability of the stem cell, in this case the oocyte, to dictate the phenotype of the surrounding stroma, whereas Risbridger and Taylor (5) create tissue chimeras in which the choice of mesenchyme dictates the fate of the stem cells, with respect to both tissue type and malignant status. The ability to induce differentiation of stem cells into a complex epithelium must represent an opportunity for research with other endocrine tissues, as seen with the breast (3), such an in vitro model, provides exciting possibilities. Thus, for instance, the study of aldosterone action, a subject of some interest to my group (8), has suffered from a lack of stable, tractable in vitro systems, which express the mineralocorticoid receptor. The construction of complex polarized sodium-transporting epithelium in vitro using stem cell technology would be a major advance. Similar examples must apply across endocrine physiology. The history of modern endocrinology is characterized by technology-driven advances that can be related to specific decades. Bioassays were followed by RIA and then radioreceptor assays, which were followed by the full molecular characterization of both the hormone and the receptor using recombinant DNA technology; this technology then expanded into functional analyses using transgenic mice and homologous recombination. One cannot help feeling that we are entering the stem cell decade in endocrine research. These four minireviews therefore will hopefully provide both insights and impetus for the further application of this rapidly maturing technology to endocrine research. First Published Online May 15, 2008