This document discusses using "letters to a friend" to assess student progress in a topic by having students write an initial letter at the beginning of a topic and a follow up letter at the end. It provides positives such as identifying misconceptions, gauging prior knowledge, and measuring engagement and progress. Potential negatives include the time needed for students to write and teachers to mark the letters. Suggestions are provided for marking the letters and future developments like increased differentiation, peer assessment, and collaboration with other departments.