Competent writing eludes many school children and adolescents, especially those with developmental language and reading disorders. Language sample analysis (LSA) of writing is a powerful but underused assessment tool that can also inform instruction. Similar to LSA of oral language, writing has been analyzed at word, sentence, and text levels using measures that include lexical diversity, sentence length, clause density, grammaticality, productivity, and text organization and content. Unique observations of writing include spelling and literate semantic and syntactic features. This article offers a review of analytic writing measures in the LSA literature from perspectives that include developmental change, language ability differences, relation to quality ratings, practical utility, and effects of genre and task. Writing samples from two 12-year-old students, with and without a language disorder, are used to illustrate application of these measures and suggest potential instruction targets.