A Computer Tutor for Writers (CTW) was designed to provide procedural facilitation to high school students while they learn the skills and knowledge associated with composition writing. Four previous year-long studies helped identify how to facilitate specific elements of the writing process. The CTW was designed to combine lessons learned from these previous studies, and provide a comprehensive support system in which students could complete classroom writing assignments. Additional factors influencing the design of the CTW were field input from high school English teachers, cognitive research into the writing process, and the cognitive apprenticeship instructional strategy. A test and evaluation of the CTW conducted during a full school-year with regular writing classes produced writing achievement gains of up to one letter grade above control groups (N = 471). Teachers and students reported that the CTW appeared to improve both the ability of students to follow a complete writing process and their ability to achieve related learning objectives. The following are appended: (1) Holistic Scoring Rubric for Expository or Persuasive Essay; and (2) ANOVA Table. (Contains 2 figures and 5 tables.)