The issue of climate migration has been gIven the image of a mass phenomenon threatening peace and security. This suggests that this issue has been subject of the process called securitisation, in which non-traditional security issues are discussed and/or acted upon in terms of security and thereby drawn into the security domain. This PhD thesis examines in what manner, to what extent, and with what political consequences climate migration is securitised, and analyses what explains the course of the process. Classical literature on securitisation assumes that securitisation is a straightforward process, with specific outcomes. Instead, I argue that the securitisation of climate migration is not clear-cut, but takes on various forms and meanings. I conduct such an analysis of securitisation by means of a flexible, contextualised and interactive framework for analysis. In this framework, I integrate the four schools of thought on securitisation: the Copenhagen School, the Paris School, Critical Security Studies, and the Risk School. The framework allows for a flexible application of their theoretical insights. It assumes that a securitisation process can take on various meanings, shaped by the context in which it is situated and by interaction processes between the actors involved. Through an analysis of context and of dynamic processes of interaction that shape the securitisation process, it becomes apparent which insights of the four schools can be applied to explain the securitisation process at hand. The securitisation of climate migration is examined in a study of the UK's Foreign & Commonwealth Office (FCO)'s securitising move on climate migration and the response to this move by the Government of India. In this study, I apply the framework for analysis to trace the diverse and complex manner in which a securitisation process develops. I show how the FCO used a security narrative on climate migration as a strategy to convince other countries, such as India, to sign up to binding mitigation targets under the UN climate convention. The FCO hoped that countries would sign up to binding mitigation targets when knowing that a failure to address climate change could result in climate migration. I examine how India has reacted to such arguments and demonstrate that it has worked in a counterproductive way. The FCO's securitising move compounded a difficult negotiation environment on climate change, and it risks legitimatising India's strict border measures to halt Bangladeshi immigration. As a final step, I review how this (unsuccessful) securitisation process affects the UK's and India's positions in climate change negotiations, and analyse how the FCO is moving towards an economic prosperity narrative to increase the effectiveness of its climate change diplomacy.