The Catalogue of Somatic Mutations in Cancer (COSMIC) is a comprehensive global resource for information on somatic mutations in human cancer. It combines curated scientific literature with tumor resequencing data from the Cancer Genome Project at the Sanger Institute. COSMIC has examined nearly 4800 genes and over 250000 tumor samples, resulting in more than 50000 mutations. The system is regularly updated every two months and provides detailed data mining capabilities through a web-based interface. This unit describes the graphical system in detail, including an example walkthrough and various ways to investigate the data by combining, respecializing queries, or viewing results in different formats. The commentary section discusses the context, issues, and potential challenges of using COSMIC, along with examples illustrating how to effectively use the data. The core workflow and interrelationships of the main pages are also outlined. Additionally, the unit covers accessing spreadsheets via FTP and exploring the Oracle database directly.The Catalogue of Somatic Mutations in Cancer (COSMIC) is a comprehensive global resource for information on somatic mutations in human cancer. It combines curated scientific literature with tumor resequencing data from the Cancer Genome Project at the Sanger Institute. COSMIC has examined nearly 4800 genes and over 250000 tumor samples, resulting in more than 50000 mutations. The system is regularly updated every two months and provides detailed data mining capabilities through a web-based interface. This unit describes the graphical system in detail, including an example walkthrough and various ways to investigate the data by combining, respecializing queries, or viewing results in different formats. The commentary section discusses the context, issues, and potential challenges of using COSMIC, along with examples illustrating how to effectively use the data. The core workflow and interrelationships of the main pages are also outlined. Additionally, the unit covers accessing spreadsheets via FTP and exploring the Oracle database directly.