The author, an experienced teacher and teacher educator, describes how she has learned to incorporate assessment purposefully into every mathematics lesson she teaches. Such an approach, she writes, tells her whether the lesson was accessible to all students while challenging the more capable, what the students learned and still need to know, how she can improve the lesson and make it more effective, and what other lesson she might offer as an alternative. The article includes strategies for assessing students' learning through written assignments and through class discussion -- for example, by asking students to explain the answers, whether or not the answers are correct; asking for more than one solution strategy; using small-group work; and asking students to restate others' ideas.