The United States' Common Core Standards for Mathematics give a prominent role for mathematical modeling in the K-12 curriculum. This has implications and creates challenges for teacher preparation programs. We describe one program's attempt to meet these challenges with a course in mathematical modeling for pre-service secondary mathematics instructors. The course was co-developed and co-taught by a mathematics educator and an applied mathematician and implemented as a design experiment. The students in the course, all mathematics majors, experienced growth in dispositions yet also faced mathematical challenges that likely extend beyond modeling contexts.