André Mathieu (1929-1968), a Canadian composer and performer, received acclaim in Europe, Canada and United States from his earliest years. He revealed brilliant pianistic skill, and critics hailed him as the "young Canadian Mozart". Mathieu's early solo piano works, written mostly in ternary form, reveal an abundance of compositional ideas based on the influences of Romantic and Impressionist styles. However, when he began composing Piano Concerto No. 3, a much larger work, his lack of compositional training became evident. Formal weakness resulted in a disconnection between themes and lack of thematic and motivic development. Not until Mathieu studied with Swiss composer Arthur Honegger did he develop and clearly display a concept of formal structure. In this study, I will demonstrate that, through compositional studies with Arthur Honegger, André Mathieu assimilated a new approach to form, resulting in the discovery of his mature compositional voice found for the first time in the Piano Concerto No. 4. The synthesis of Romantic and Impressionistic styles in this concerto further contributes to the unique nature of this mature voice.This document is organized in six chapters. Chapter I reviews current literature on André Mathieu and his music and points out both the values and the limitations of this literature. Chapter II relates Mathieu's biography. Chapter III provides background information on Mathieu's solo piano works and four concerti, as well as the difficult pianistic techniques composed into those works. Chapter IV discusses the influences of Romantic and Impressionist styles in Piano Concerto No. 4 and how Mathieu synthesized both styles. Chapter V analyzes the form, motivic and thematic material, and rhythmic design of all three movements of Piano Concerto No. 4 and compares them with that of Piano Concerto No. 3. Honegger's influence also is discussed in Chapter V. Chapter VI is the conclusion. Musical examples throughout will illustrate points made.