Messiaen rarely commented on form. His posthumously published seven-volume Traité de rythme, de couleur, et d’ornithologie offers an important new perspective on many aspects of his music; yet despite the vast amount of information that the composer’s surveys of his works disclose – details that, in many cases, only he himself could have supplied – his analyses do have certain limitations, notably with regard to form. In a largely complimentary essay on Messiaen in La Musique depuis Debussy (1961), André Hodeir closes with a scathing assessment of Messiaen’s activity in the domain of form. In order to assess the validity of Hodeir’s comments, I have surveyed Messiaen’s analyses, proposed a hypothetical taxonomy of formal models in a range of the composer’s works, and attempted to place his thinking in the wider context of attitudes to form in the twentieth century.