New generations of DNA sequencing technologies are enabling the systematic study of genetic derangement in cancers. Sequencing of cancer exomes or transcriptomes or even entire cancer genomes are now possible, though technical and economic challenges remain. Cancer samples are inherently heterogeneous and are often contaminated with normal DNA, placing additional demands on informatics tools for detecting genetic variation. However, even low coverage sequencing data can provide valuable information on genetic rearrangements, amplifications and losses in tumor genomes. Novel recurrent oncogenic mutations and fusion transcripts have been discovered with these technologies. In some sequenced cancer genomes, tens of thousands of genetic alterations have been discovered. While this enables the detailed dissection of mutation classes, it also presents a formidable informatics problem of sorting active driver mutations from inactive passenger mutations in order to prioritize these for further experimental characterization.