As a music theorist, Theisen specializes in the analysis and pedagogy of 20th-century and contemporary classical music. He has presented research at academic conferences on the music of Elliott Carter, Gyorgy Ligeti, Pierre Boulez, and Frank Martin. He is currently writing a book on the late music of Tristan Keuris.
Read: PhD dissertation (Music Theory, Florida State Univ., 2010) A Multifaceted Approach to Analyzing Form in Elliott Carter's Boston Concerto
Read: review of Brian Alegant's book The Twelve-Tone Music of Luigi Dallapiccola in Music Theory Online 19.1
Read: a brief analytical essay on Elliott Carter's song "Una Colomba"
Read: What's the Big Deal about All-Interval Tetrachords and the All-Triad Hexachord?
Read: Text, Harmony, and Transformation in Frank Martin's Sechs Monologe aus Jedermann
Theisen has served as engraver of the 8th, 9th, and (forthcoming) 10th Editions of the best-selling textbook Music for Sight Singing by Nancy Rogers and Robert Ottman.