In order to teach writing effectively, teachers need assessment tools that work for diagnostic purposes--tools that can help them identify students' specific strengths and challenges with writing, as well as generate new ideas for instruction. This study explored what 5 high school teachers (3 ELA and 2 ESL) learned about their students' strengths and challenges as writers from the students' performance on a think-aloud-protocol (TAP) Assessment, how this information differed from what they knew about the students as writers based only on their previous writing in class, and what ideas this new TAP-generated information gave them for instruction. We found that the TAP Assessment was especially useful in generating new information related to audience awareness, interpretation of the task, and self-evaluation. Teachers also offered creative suggestions for adapting the TAP Assessment for further use in their classrooms.