Co-teaching is a special education service delivery model in which a general educator and special educator work together to plan and teach a class comprising students with and without disabilities. The co-teaching model has increased in popularity and is now a common fixture in many schools. However, observational research indicates co-teaching may not be used effectively in many cases; special educators are often not directly involved in providing instruction, instead essentially serving as assistants to the general educator. One possible reason for special educators’ lack of integration into instruction could be confusion about their instructional role. The purpose of co-teaching is to provide access to the general curriculum for students with disabilities while also providing them specially designed, individualized instruction (SDI). However, SDI is not well understood, and in secondary classes particularly, it can be difficult to find time within the class to provide it. Lesson planning for secondary mathematics courses poses a particular problem for special educators, many of whom are experts in mathematics content. In this article, we discuss what SDI should include in a secondary mathematics course and propose ways in which co-teachers can incorporate SDI into their classes.