This chapter aims to identify sources of individual differences in writing proficiency, and ways to measure these differences. The construct of writing proficiency is delineated and embedded in cognitive approaches to writing research. It is considered to be a multi-faceted construct, which implies that various component skills contribute to the quality of the eventual writing performance. Writing in a second language often builds on L1 writing skills and procedures, but also raises its own questions, such as the role of transfer of (metacognitive) knowledge, and the writer’s strength and weaknesses in the target language. The best way to assess a language learner’s writing proficiency very much depends on the assessment purposes, and can range from a focused measurement of subskills with discrete-point tests, for diagnostic purposes, to a general performance assessment with holistic rating, for admission purposes. A number of characteristics of these measurements are discussed in the second part of this chapter.