For a decade, the technologies behind DNA sequencing have improved rapidly in cost reduction and speed. Sequencing in large populations of cancer patients is leading to dramatic advances in our understanding of the cancer genome. The wide variety of cancer types sequenced and analyzed using these technologies has revealed many novel fundamental genetic mechanisms driving cancer initiation, progression, and maintenance. We have deepened our understanding of the signaling pathways, demonstrating disruption in epigenetic regulation and destabilization of the splicing machinery. The molecular mechanisms of resistance to targeted therapies are being elucidated for the first time. The translation of genome-scale variation into clinically actionable information is still in its infancy; nevertheless, insights from sequencing studies have led to the discovery of a variety of novel diagnostic biomarkers and therapeutic targets. Here, we review recent advances in cancer genomics and discuss what the new findings have taught us about cancer biology and, more importantly, how these new findings guide more effective diagnostic and treatment strategies.