More than 10 years after the release of the original Guidelines for the Assessment and Instruction in Statistics Education (GAISE) College Report, the proposed reforms have been widely embraced by the statistics education community for introductory statistics courses. Although the report is geared toward the introductory course, the GAISE recommendations are just as relevant for higher level statistics courses. We will discuss implementing the reforms in the two-semester calculus-based undergraduate mathematical statistics sequence. While all of the GAISE recommendations are applicable, we will focus on methods for incorporating conceptual understanding and active learning into these courses, which are typically taught in a traditional lecture style. The course revision does not require substantial changes in course content, so the necessary theoretical content is not sacrificed. We will describe activities and teaching strategies we have used to promote student understanding and problem solving, as well as share student reflections on the course revisions.