Cancer stem cells (CSCs) have received considerable attention from the research community since they were first reported in human acute myeloid leukemia 15 years ago. Accumulating evidence suggests that CSCs are responsible for tumor initiation and progression, drug resistance, and metastasis in both liquid and solid tumors. These findings lead to the development of novel compounds targeting CSC populations that is becoming increasingly important for eradicating CSCs in heterogeneous tumor masses and to cure the cancer. Since 2003, we have participated in CSC studies and encountered crucial early events in the field. This article reviews the history of CSC biology, clarifies the term and its definition, and further addresses the issue of how to utilize CSCs in therapeutic target discovery and drug development based on our substantial experience.