This volume provides a state-of-the-art overview of theory, methodology and practices in the assessment of writing. The focus throughout the book is on the construct of writing and its assessment: what constitutes writing ability and how can it be defined (in various contexts)? This question cannot be answered without looking into the methodological question of how to validate and measure the construct of writing ability. Throughout the book, therefore, discussions integrate theoretical and methodological issues. A number of chapters discusses whether varying definitions and varying operationalizations of writing ability are needed in various contexts, such as formative assessments versus summative assessments, large scale assessments versus individual assessments, different tasks, different genres, and different languages, but also different age groups. A range of rating methods is investigated and discussed in this book. The ongoing debate on holistic versus analytic ratings, and the different underlying conceptions of writing proficiency, is a pertinent matter, on which a number of chapters in this volume shed new light. The matter is discussed and analyzed from various angles, such as generalizability of judgements and usability in formative contexts. Another fundamental debate concerns computer scoring of written products. A nuanced discussion of its validity is presented in this volume.