Autophagy is a catabolic program that is responsible for the degradation of dysfunctional or unnecessary proteins and organelles to maintain cellular homeostasis. Mechanistically, it involves the formation of double-membrane autophagosomes that sequester cytoplasmic material and deliver it to lysosomes for degradation. Eventually, the material is recycled back to the cytoplasm. Abnormalities of autophagy often lead to human diseases, such as neurodegeneration and cancer. In the case of cancer, increasing evidence has revealed the paradoxical roles of autophagy in both tumor inhibition and tumor promotion. Here, we summarize the context-dependent role of autophagy and its complicated molecular mechanisms in the hallmarks of cancer. Moreover, we discuss how therapeutics targeting autophagy can counter malignant transformation and tumor progression. Overall, the findings of studies discussed here shed new light on exploiting the complicated mechanisms of the autophagic machinery and relevant small-molecule modulators as potential antitumor agents to improve therapeutic outcomes.