PhD dissertation. Abstract Design thinking is a collaborative and human centred approach to solving problems. Over the past decade design thinking has evolved considerably, particularly with regard to innovation within the sectors of design and business. Despite this sharp rise to popularity there remains limited understanding of how design thinking is applied in practice and little empirical investigation into this subject. Without this understanding further informed application and development of the approach will be hampered. The ‘design led professional’ is an individual who uses design approaches in their work practices whose education and experience however may not necessarily be in design. The central aim of this thesis is to understand how the ‘design led professional’ applies design thinking in practice with large organisations where the focus is on designing intangible products such as systems, services and experiences. The thesis addresses the research problem through the exploration of the question: How does the design led professional understand and enact design thinking in practice? This question is explored within the context of the design led professional working with large organisations. A qualitative research approach was adopted, which involved ethnographic methods of semi structured interviews, artefact analysis and participant observation. Data was collected across three studies: an expert interview study, a retrospective case study and a participatory case study. The constant comparative grounded theory method was used to analyse and synthesise data. Research findings, contextualised within relevant literature, reveal the composition of design thinking in practice: as constrained by the approach taken in applying design thinking; the maturity of the design led professional and the environment in which design thinking is conducted. On this basis two models are proposed in the conclusion as a foundation for further application and development. The first presents a scale of design thinking maturity based upon two perspectives of design thinking as a way of work and a way of life. The second model maps the interdependent relationship between the three components of design thinking in practice of the approach, the design led professional and the environment in which it is conducted. The evidence generated through this research provides a framework to assist the public and those who practice design thinking to better understand and articulate design thinking. In addition it provides a foundation for further empirical research that explores the realistic application of design thinking in practice and the critical role of the design led professional.