Writing is a critical 21st century skill. Today's knowledge economy places a premium upon collaboration and written communication, which means that the skilled writer enters the job market at a significant advantage (Aschliman, 2016; Brandt, 2005). And yet students typically enter the job market with weak writing skills. Only 27% of 12th-grade students demonstrated proficiency in writing on the National Assessment of Educational Progress (The Nation's Report Card, 2022). Similarly, according to one recent study, only 27% of employers classified recent college graduates as "well-prepared" for written communication in the workplace (Stewart et al., 2016). Disparities in writing skills both reflect and contribute to inequities in our society. Certain groups, (e.g., people who identify as White and female) typically display stronger writing achievement, at least on the kinds of writing tasks that are highly valued in academic institutions and the professions (Roberts et al., 2017), with evidence that this leads to further inequities (Warren, 2013). Writing matters, and thus it is important to understand why so many students experience writing as deeply challenging and how they can be encouraged to develop their writing skills. For decades, writing has been a major focus of research for the ETS Research and Development (R&D) division. The results of our efforts include substantial contributions to the measurement and development of student writing skills, with important implications for teacher professional development and the improvement of teaching practice.