The diagnostic assessment of writing is an important aspect of language testing which has often been neclected in the literature. However, it is an area which poses special challenges to practioners both in the classroom and in large-scale testing situations. This book presents a study which set out to develop and validate a rating scale specifically designed for the diagnostic assessment of writing in an academic English setting. The scale was developed by analysing a large number of writing performances produced by both native speakers of English and learners of English as an additional language. The rating scale was then validated using both quantitative and qualitative methods. The study showed that a detailed data-based rating scale is more valid and more useful for diagnostic purposes than the more commonly used impressionistic rating scale.