The Corpus of Contemporary American English is the first large, genre-balanced corpus of any language, which has been designed and constructed from the ground up as a ‘monitor corpus', and which can be used to accurately track and study recent changes in the language. The 400 million words corpus is evenly divided between spoken, fiction, popular magazines, newspapers, and academic journals. Most importantly, the genre balance stays almost exactly the same from year to year, which allows it to accurately model changes in the ‘real world'. After discussing the corpus design, we provide a number of concrete examples of how the corpus can be used to look at recent changes in English, including morphology (new suffixes –friendly and –gate), syntax (including prescriptive rules, quotative like, so not ADJ, the get passive, resultatives, and verb complementation), semantics (such as changes in meaning with web, green, or gay), and lexis––including word and phrase frequency by year, and using the corpus architecture to produce lists of all words that have had large shifts in frequency between specific historical periods.