Stem cell research represents an area with great scientific and therapeutic promise. Because stem cells are unique in their ability to self-renew and to differentiate into multiple cell types, they can be used to elucidate normal cell processes as well as to understand the diseased condition. In addition, stem cells from individuals with various diseases can be utilized for biotechnology advance, for example, in drug screening. There is a widespread hope that these cells will one day be used to treat human injury and disease. The articles in this collection represent a small sampling of exciting advances in stem cell biology as they have been presented in Science magazine over the past few years.