Hepatocellular carcinoma (HCC) and cholangiocarcinoma (CCC) are the most common liver tumors and a leading cause for cancer-related death in men. Notch2 regulates cellular differentiation in the developing and adult liver. Although aberrant Notch signaling is implicated in various cancers, it is still unclear whether Notch2 regulates proliferation and differentiation in liver carcinogenesis and thereby contributes to HCC and CCC formation. Here, we investigated the oncogenic potential of constitutive Notch2 signaling in the liver. We show that liver-specific expression of the intracellular domain of Notch2 (N2ICD) in mice is sufficient to induce HCC formation and biliary hyperplasia. Specifically, constitutive N2ICD signaling in the liver leads to up-regulation of pro-proliferative genes and proliferation of hepatocytes and biliary epithelial cells (BECs). Using the diethylnitrosamine (DEN) HCC carcinogenesis model, we further show that constitutive Notch2 signaling accelerates DEN-induced HCC formation. DEN-induced HCCs with constitutive Notch2 signaling (DENN2ICD HCCs) exhibit a marked increase in size, proliferation, and expression of pro-proliferative genes when compared with HCCs from DEN-induced control mice (DENctrl HCCs). Moreover, DENN2ICD HCCs exhibit increased Sox9 messenger RNA (mRNA) levels and reduced Albumin and Alpha-fetoprotein mRNA levels, indicating that they are less differentiated than DENctrl HCCs. Additionally, DENN2ICD mice develop large hepatic cysts, dysplasia of the biliary epithelium, and eventually CCC. CCC formation in patients and DENN2ICD mice is accompanied by re-expression of hepatocyte nuclear factor 4α(HNF4α), possibly indicating dedifferentiation of BECs. Conclusion: Our data establish an oncogenic role for constitutive Notch2 signaling in liver cancer development. (HEPATOLOGY 2013)