PURPOSE OF REVIEW: The last years of cancer research have established the concept of cancer stem cells (CSCs) as a subpopulation of cells within a tumor entirely responsible for tumorigenesis. This has aroused expectations that targeting cancer stem cells would allow effective tumor eradication. This review aims to summarize the relevant achievements in the field and to highlight the complex mechanisms that are involved in regulating CSC function. RECENT FINDINGS: A growing number of studies have identified CSCs in a variety of human tumor types. The focus of attention is now moving to discover molecular signals which are essential to sustain CSC. We summarize recent results on intrinsic and extrinsic signaling pathways such as Wnt signals, which control stem cell self-renewal and highlight the role of the microenvironment or niche in this process. SUMMARY: The discovery of cancer stem cells points into new directions to gain better understanding of cancer biology. This is expected to alter the design of clinical research programs and to improve the way we assess the efficacy of novel anticancer drugs. Several conclusions and predictions derived from this concept hold great promise to speed up the process of discovering effective targets for clinical application