This issue: Trigonometric Functions. Connecting Research to Teaching aims to bridge the gap between research and practice by describing current research and then demonstrating its importance and applicability to practicing classroom teachers. The article presents lessons learned from research investigating the learning and teaching of trigonometric functions. This department presents information to help teachers understand students’ conceptions or misconceptions of important ideas, consider various approaches to teaching, and offer activities that probe students’ understanding. Connecting Research to Teaching is devoted to articles that bring research insights and findings to an audience of teachers and other mathematics educators (supervisors, professional developers, and teacher educators), and is a regular department of Mathematics Teacher.