USING TRIGONOMETRY AS AN EXAMPLE, A REPORT ON THE USE OF THE OVERHEAD TRANSPARENCY PROJECTOR AS AN AUDIOVISUAL AID IN CONCEPT BUILDING IS PRESENTED. SOME ADVANTAGES OVER THE CONVENTIONAL CHALKBOARD METHOD ARE--(1) OVERLAYS MAY BE SO ATTACHED THAT ANY PROBLEMS OR DIAGRAM PROCEEDS FROM THE SIMPLE TO THE COMPLEX BEFORE THE STUDENT'S EYES. (2) FACING THE CLASS, THE INSTRUCTOR CAN BE MORE CERTAIN OF APPROPRIATE TEMPO, THERE IS NO LONG DELAY WHILE THE INSTRUCTOR WRITES ON THE BLACKBOARD. (3) EXTRANEOUS MATERIAL PREVIOUSLY APPLIED IS EASILY COVERED SO THAT ALL ATTENTION IS CONCENTRATED ON THE IDEA UNDER DISCUSSION. (4) A LIGHT IS FOCUSED ON THE SCREEN, WHICH IS A PSYCHOLOGICAL AID IN FOCUSING ATTENTION. (5) OVERLAYS, PROPERLY ATTACHED, MAKE POSSIBLE INSTANT AND COMPLETE REVIEW OF ALL OR ANY PART OF ANY LESSON. (6) INSTRUCTOR TIME SPENT IN LESSON PREPARATION IS DECREASED. OVERHEAD PROJECTION METHODS AID IN CONCEPT BUILDING BUT DO NOT SUBSTITUTE FOR STUDENT ABILITY AND EFFORT. ABSTRACT THINKING SHOULD BE DEVELOPED ONCE CONCEPTS HAVE BEEN BUILT THROUGH AUDIOVISUAL METHODS, AND CARE SHOULD BE TAKEN TO AVOID COMPLETE DEPENDENCE ON PHYSICAL AIDS. THIS ARTICLE WAS PUBLISHED IN "TEACHING TOOLS," 1958.