Behavioural design has emerged as an important domain of design research and practice. However, there is a need to better distinguish behavioural design and subsequently map its unique features and challenges. In answer to this need, this work examines the evolving role of behaviour in design and contrasts this with eleven in-depth behavioural design cases. This has resulted in our identification of behavioural design as a new paradigm of design, with a number of unique characteristics. Based on this, we propose a model of behavioural design. Furthermore, we identify three critical barriers to behavioural design in practice, and suggest ten mitigating enablers. Together these findings provide implications for further development of theory, practice, and education in behavioural design.