Writing is a highly valued skill that is often neglected in the classroom; one reason is that teachers often do not receive adequate training in writing assessment and instruction. Teachers, particularly preservice teachers, need practice making detailed assessments of student writing and to build their confidence for assessing student writing, but practical issues of time and resources often constrain the frequency and quality of training they receive. This mixed method study focused on the design and evaluation of an online tool for building preservice teachers' writing assessment skills and self-efficacy for writing assessment. In the study, teacher education students interacted with actual 4th-graders' writing samples via a Web-based critical thinking tool. They received scaffolded practice in assessing multiple student papers and justified their assessments using analytic criteria. After each paper, they received feedback that included access to expert assessments and those of their peers, along with both teacher and peer rationales for their ratings. Participants significantly improved in ability to accurately assess student writing using an analytic approach and in knowledge of the writing traits. They also showed significantly greater self-efficacy for assessing student writing and high levels of satisfaction with the Web-based tool and their overall learning experience. (Contains 5 tables and 6 figures.)