Partnerships have become increasingly prevalent across a wide range of sectors for the delivery of services and implementation of policy. Partnerships are seen as a more inclusive way of delivering policy interventions than more traditional state led or 'top-down' approaches. Yet it is unclear as to whether the partnership approach provides an effective vehicle for policy delivery. Academic debate has also focused on the most appropriate methods for evaluating different types of partnerships. To date, effectiveness has been viewed as an endpoint; a cumulative process which is the product of a set of variables acting on a one-dimensional process, which results in the ability of a partnership to achieve its goals. Concerns about the efficacy of partnership intervention, and the difficulty in evaluating the effectiveness of partnerships, are of increasing relevance to coastal environments, as policy implementation has begun to depend more heavily on partnerships in order to achieve sustainable development objectives in this geographical area. This thesis seeks to advance current understanding of the effectiveness of partnerships. Specifically, it explores the concept of effectiveness through an in-depth study of partnerships involved in managing marine nature-based tourism. Based on analysis of documents, minutes and interviews, the achievement of key determinants of effectiveness was traced through time in three case study partnerships. The results indicated that effectiveness is more accurately conceptualised as a composite, comprised of a set of determinants which may be internal or external to the partnership. Moreover, through the development of individual narratives tracing partnership evolution and development, this study has also shown that determinants of effectiveness can and do change over time. Although the results of this study relate to marine nature-based tourism partnerships, the findings have important implications for the design and implementation of partnerships in other policy environments; in the way that partnerships are initiated and managed, and also in the way that they are evaluated.