Autophagy is an intracellular degradative process that occurs under several stressful conditions, including organelle damage, the presence of abnormal proteins, and nutrient deprivation. The mechanism of autophagy initiates the formation of autophagosomes that capture degraded components and then fuse with lysosomes to recycle these components. The modulation of autophagy plays dual roles in tumor suppression and promotion in many cancers. In addition, autophagy regulates the properties of cancer stem-cells by contributing to the maintenance of stemness, the induction of recurrence, and the development of resistance to anticancer reagents. Although some autophagy modulators, such as rapamycin and chloroquine, are used to regulate autophagy in anticancer therapy, since this process also plays roles in both tumor suppression and promotion, the precise mechanism of autophagy in cancer requires further study. In this review, we will summarize the mechanism of autophagy under stressful conditions and its roles in tumor suppression and promotion in cancer and in cancer stem-cells. Furthermore, we discuss how autophagy is a promising potential therapeutic target in cancer treatment.