This article presents an analysis of Instant Messaging (IM), a one-to-one synchronous medium of computer-mediated communication. Innumerable articles in the popular press suggest that increasing use of IM by teens is leading to a breakdown in the English language. The analyses presented here are based on a unique corpus involving 72 teenagers and over a million words of natural, unmonitored IM. In addition, a corpus of speech from the same teenagers is examined for comparison. Targeting well-known IM features and four areas of grammar, we show that IM is firmly rooted in the model of the extant language. It reflects the same structured heterogeneity (variation) and the same dynamic, ongoing processes of linguistic change that are currently under way in contemporary varieties of English. At the same time, IM is a unique new hybrid register, exhibiting a fusion of the full range of variants from the speech community—formal, informal, and highly vernacular.