There is no comprehensive model of language test or examination impact and how it might be investigated within educational contexts by a provider of high-stakes examinations, such as an international examinations board. This thesis addresses the development of such a model from the perspective of Cambridge ESOL, a provider of English language tests and examinations in over 100 countries. The starting point for the thesis is a discussion of examinations within educational processes generally and the role that examinations board, such as Cambridge ESOL play within educational systems. The historical context and assessment tradition is an important part of this discussion. In the literature review, the effects and consequences of language tests and examinations are discussed with reference to the better known concept of washback and how impact can be defined as a broader notion operating at both micro and macro levels. This is contextualised within the assessment literature on validity theory and the application of innovation theories within educational systems. Methodologically, the research is based on a meta-analysis which is employed in order to describe and review three impact projects. These three projects were carried out by researchers based in Cambridge to implement an approach to test impact which had emerged during the 1990s as part of the test development and validation procedures adopted by Cambridge ESOL. Based on the analysis, the main outcome and contribution to knowledge is an expanded model of impact designed to provide examination providers with a more effective “theory of action”. When applied within Cambridge ESOL, this model will allow anticipated impacts of the English language examinations to be monitored more effectively and will inform on-going processes of innovation; this will lead to well-motivated improvements in the examinations and the related systems. Wider applications of the model in other assessment contexts are also suggested.