Pancreatic ductal adenocarcinoma (PDAC) is still a devastating and incurable disease with a median survival of 3-6 months and a 5-year survival rate of 1-4% when all stages are considered. Although crucial advances in our understanding of the molecular pathogenesis of the disease have been made, the exceptional aggressiveness of PDAC remains largely unexplained. Some key results will probably direct future PDAC research activities. For example, recent identification of pancreatic tumor stem cells has stimulated the debate over the cell of origin. Further, powerful new genetically engineered mouse models support the concept that stepwise progression of epithelial precursor lesions leads to invasive PDAC as a result of accumulating mutations in K-ras, INK4A/ARF, TP53 and DPC4; these models accentuate the initiating function of the K-ras mutation. Established PDAC exhibits all the classic hallmarks of cancer, including self-sufficiency in growth signals, insensitivity to anti-growth signals, evasion of apoptosis, limitless replicative potential, sustained angiogenesis, tissue invasion, and metastasis. This review provides an overview of the molecular machinery that PDAC utilizes to acquire these tumorigenic capacities. Moreover, recent advances have identified essential elements of key pathways partly recapitulating developmental signals, and of the tumor microenvironment that promotes tumor growth through the complex interplay of its different cellular components. In spite of progress in molecular research, there is still a dichotomy between the encouraging results obtained with targeted interference of numerous oncogenic pathways in vitro and a lack of significant improvement in clinical detection and survival. Thus our primary challenge remains to translate the solid knowledge of genetic and epigenetic alterations in PDAC into clinical tools which can be used for early diagnosis and effective therapy.