We examined the contributions of English proficiency, genre, and the use of textual sources to adolescent writing. The sample included 1819 native English speakers and language minority students from 127 seventh- and eighth-grade classes in an urban school district. Students were randomly assigned one of three source-based essay prompts (narrative, explanatory or argumentative) as part of the annual state assessment, and all students wrote a common, on-demand argumentative essay that did not require the use of textual sources. Overall, language minority students who were fluent English proficient wrote higher quality nonsource-based essays that contained more difficult vocabulary, were better structured, and used examples and details more effectively than native English speakers. They also outperformed native English speakers in source-based writing. Students with limited English proficiency showed weaker performance on both source-based and nonsource-based writing tasks. Differences in English proficiency held across genres. Students who wrote argumentative essays obtained higher ratings than those who wrote narrative or explanatory essays. Source-based and nonsource-based writing were moderately correlated. Regression analyses revealed that in addition to English proficiency and genre, reading comprehension's contribution to source-based writing was almost double that of nonsource-based writing. Implications of the findings for theory and practice are discussed. [This is the online version of an article published in "Reading and Writing."]