Today, cancer is the second most prevelant cause of death, after heart disease, in the industrialized world. Till now, and despite promising results in cancer research, the primary cause of death in cancer is still 1: the formation of metastases at distant organs sites, 2: the presence of cancer cells that are resistant to current cancer therapies (chemotherapy/radiation) and which cause cancer relapse. Within the past three years it became evident that also solid tumor tissues contain a kind of stem cells, so-called cancer cells. These cancer stem cells have been described as chemo-/radiation therapy resistant tumor initating cells. The purpose of this book is to summarize the latest findings about the role of stem cells in cancer.