Legitimacy is widely accepted as an important resource for an organisation, strategy, or individual to possess. However, the process of gaining legitimacy has received limited attention in the academic literature. This thesis examines the strategies and actions that individuals employ in the process of legitimising their sustainability strategy within an organisation. Based on semi-structured interviews with 51 Heads of Sustainability, the research extends the existing ‘conformance, selection, manipulation’ legitimising strategy model, becoming one of the first to demonstrate how these legitimising strategies are interrelated both concurrently and temporally. It finds that multiple legitimising strategies are used simultaneously by individuals. Moreover, a pattern emerges whereby individuals begin with conformance-only legitimising when sustainability has limited integration, but employ all three legitimising strategies where sustainability integration is extensive. In addition to this, the research articulates two specific categories of actions that are used by individuals in the process of deploying these umbrella legitimising strategies: framing and developing coalitions of support. Framing actions comprise micro-reframing, disassociation, contextualisation, analogy, and differentiation and personalisation. Developing coalitions of support actions comprise leveraging sponsorship, networking, enhancing employee engagement, and continually promoting. From this empirical research a generalised legitimising pathway is proposed which demonstrates the progression of legitimising from using conformance-only through to using all three legitimising strategies, and the actions employed by the individual in these different stages.