The notion of harmonic ‘function’ was introduced by Riemann in 1891 to describe aspects of tonal music, but, as musical language evolved, its meaning and application developed beyond the premise of tonality. This study re-evaluates the relevance of harmonic functionality for composers today, through a reassessment of what may constitute functionality in post-tonal music and a practical exploration of the concept in ten new compositions. The general openness of harmonic systems since the 1980s has led composers across a variety of aesthetic standpoints to often combine diverse types of harmonic material in one same piece; this thesis argues that, within such a framework, harmonic functionality is potentially an important instrument of coherence, strength and inventiveness in the musical argument. The first part examines the main terminology, including the notions of ‘harmony’, ‘post-tonality’ and the origin and development of the idea of ‘harmonic functionality’. Several composers and theorists are considered, with a particular focus on Riemann. The second part centres on the ten compositions, particularly on Nachtmusik, with both detailed and large-scale analysis aiming to clarify how each piece informed specific aspects of the study. For the sake of comparison, aspects relating to harmonic functionality in other composers’ work are also discussed, namely in Stravinsky’s Requiem Canticles and Adès’ Polaris and Tevot. It is concluded that harmonic functionality is generally related to formal functions, and that the combined action of pitch and other parameters establishes functional properties very much depending on musical context. Certain elements play a central role in the definition of functionality: 1) sense of direction; 2) recurrence and change; 3) hierarchy; 4) pitch polarity; 5) extra-musical meaning. The creation of harmonic functionality in post-tonal music is a problem which composers often deal with intuitively in their practice, and the notion is still frequently considered in a strictly tonal sense by theorists; through the clarification and discussion of the process in post-tonality, its potential is likely to be more fully explored, both theoretically and practically.