What is a musical idea? What are its constituents - intervals, rhythms, or perhaps utterly different entities? How can a musical idea serve as a basis for musical structure, process, and affect? What distinguishes different theories of motivic and thematic structure from each other? What are the limitations of such theories, and how can their use enhance our (musicians, researchers, listeners) understanding of music? The seminar discusses these issues, starting with an inquiry of basic methodological queries concerning motivic and thematic analysis, and continuing by presenting several leading analytical approaches, demonstrated through analyses of tonal and post-tonal music.