Since the early 20th century, when Hans Spemann first twinned salamander embryos, scientists have made astounding progress in the science and technology of cloning. They have now developed the means to apply cloning in research, agriculture, and medicine. With advancements in cloning techniques for stem cell research, scientists have been able to explore human diseases at a cellular level, attempting to better understand the cellular mechanisms involved in disease. Readers explore the history, science, applications, and ethical issues of cloning. Sidebars profile pioneers in the field, including John Bertrand Gurdon, Ian Wilmut, Shinya Yamanaka, and James Thomson.