This article explores the processes of writing in science and in particular the "complex performance" of writing a scientific argument. The article explores in general terms the nature of scientific argumentation in which the author-scientist makes claims, provides evidence to support these claims, and develops chains of scientific reasoning to coordinate claims and evidence. The article then describes a case in which two classes of Grade 8 students in a New York City school wrote scientific arguments in a web-writing and peer review environment that provides each writer with "as-you-go" formative assessment on the constitutive elements of their arguments.