Retrofit programmes for installing energy efficient technologies in social housing are a key part of efforts to reduce UK carbon emissions by 80 per cent by 2050. This requires a reduction in CO2 emissions by an average of 80%, from all housing, in order to assist the UK’s long term goals. The UK’s turnover of housing stock is relatively slow compared to most developed countries and approximately 87% of the current housing stock will still be standing in 2050. Therefore, to meet carbon emissions targets, existing buildings must be refurbished or ‘retrofitted’ with technologies which reduce carbon emissions on a huge scale. The Government intends to achieve this by driving energy efficiency in households and businesses predominantly through the proposed Green Deal framework. This represents a shift in policy approach since the 2010 elections, towards a private finance and private industry approach, as rather than the previous Labour Governments predominately state managed and grant-aided social retrofit approach. The influence of the economic recession at the time of this transition is also likely to be a key driver of the Governments changing approach to financing the retrofit of millions of UK homes. Other strategies such as the UK Fuel Poverty Strategy are also intended to dove-tail with this national push to retrofit housing stock, due to reduced energy costs and increased thermal comfort. There is great potential for the proposed national retrofit mobilisation to reduce carbon emissions from homes, contribute to economic growth and provide other benefits such as the reduction of Fuel Poverty. However, the amount of energy used in homes is largely dependent on the behaviours of the occupier(s) and occupant behaviour can determine the effectiveness of retrofit programmes and thus impact on the potential of this significant mobilisation to reduce energy consumption and carbon emissions from housing. Thus, occupant behaviour is increasingly recognised as a critical element to be acknowledged and addressed in order to meet carbon reduction targets, both within the literature (excluding the policy literature) by and those delivering retrofits on the ground. This research provides a unique insight into occupant energy use behaviour by evaluating a ‘live’ project to retrofit energy efficient being implemented by Gentoo Group which includes construction and social landlord roles. The literature review relevant to the research focuses on Psychological theories of behaviour and Practice Theory. This provides insights from both paradigms provide two viewpoints on behaviour: an insight into the nature of individual behaviour (Psychological theories of behaviour), and; a consideration of how the framework and structure of society (including aspects such as technology) interacts with the individual’s practices (Practice Theory). The research methodology utilised an inductive approach, underpinned by a qualitative research design. In-depth interviews were conducted before and after specific interventions (a technical intervention and informational interventions) took place, these interviews were recorded and transcriptions were thematically organised and analysed using the template analysis technique. This process first identified ‘patterns of behaviour related to energy use’ arising due the project interventions and secondly based on the frequency of these occurring, identified ‘key patterns’. The theoretical perspectives of the Psychology and Practice Theory literature were drawn on in order to contextualise the findings of the research, but it this thesis does not attempt to apply them in an empirical approach. The analysis process instead draws on specific elements from both of the disciplines to assist the interrogation of the ‘key patterns’ so they may be better explained or understood. Key findings of the research highlighted that energy use behaviour is impacted by the introduction of technology, and tenant behaviour can potentially impact on the energy saving effectiveness of retrofit projects. Significant impacts were identified specifically where tenants had an interaction with the technology being introduced and the informational intervention had no significant impact on behaviour. ‘Key patterns’ indicated several factors which influence tenant energy use behaviour and of these the barriers to retrofit effectiveness were identified as: limited access to knowledge and skills; habits preventing behaviour change to utilise introduced technologies; the quality of installation and function of the technical intervention; convenience of introduced technology potentially increasing energy use, and: the need or desire for thermal comfort. The thesis concludes that energy use behaviour is pivotal factor in determining retrofit effectiveness and that behaviour, and in particular behaviour-related barriers to retrofit effectiveness, should be acknowledged and addressed as part of the UK retrofit strategy, especially in the light emerging policies such as the Green Deal and Energy Company Obligation, which intend to drive retrofit on a huge scale. Recommendations are made inform retrofit practitioners and academic and policy debates on behaviour in the context social housing retrofit, and suggestions are made for future research to explore this research area further.