The study of RUNX3 in tumor pathogenesis is a rapidly expanding area of cancer research. Functional inactivation of RUNX3-through mutation, epigenetic silencing, or cytoplasmic mislocalization-is frequently observed in solid tumors of diverse origins. This alone indicates that RUNX3 inactivation is a major risk factor in tumorigenesis and that it occurs early during progression to malignancy. Conversely, RUNX3 has also been described to have an oncogenic function in a subset of tumors. Although the mechanism of how RUNX3 switches from tumor suppressive to oncogenic activity is unclear, this is of clinical relevance with implications for cancer detection and prognosis. Recent developments have significantly contributed to our understanding of the pleiotropic tumor suppressive properties of RUNX3 that regulate major signaling pathways. This review summarizes the important findings that link RUNX3 to tumor suppression.