This thesis develops a 'human economy' approach to understanding economic life thatelucidates the social nature of economic reason. It explores deep structural changes infinancial capitalism through the emergence of the sustainability paradigm in institutionalinvestment, which involves the integration of environmental and social factors and longtermthinking into mainstream financial corporate valuations. The research is based on anextended-case study through participant-observation with one sustainable investmentagency. The company is led by a power figure in sustainable finance and his trustednetwork of elite actors, who aim to be at the vanguard of the changes in institutionalinvesting as they construct the category of the sustainable investor. The thesis explores theambiguities inherent to such an undertaking and intends to open up new ground foreconomic anthropology and the anthropology of finance.The ethnography shows how the investment agency developed from a start-up firm withpeople operating from their homes to an established organisation in London. The majorityof research was conducted with a team of sustainable investment analysts whose role it isto produce ratings on companies and influence the decision-making of financial analystsand portfolio managers. The ethnography depicts the everyday practices of this team, howthe material arrangements of the investment agency were constructed, and actors' attemptsto develop relationships with financial experts within investment processes.The findings are used to critique institutional investing and comment on normative andpolicy changes in the industry that centre on the figure of 'the fiduciary'. The thesis alsopoints to new areas for research such as the links between corporate executives andsustainable investors. A historical account of investment management is also presented asa way of deconstructing many of the logics and ideas that were encountered duringfieldwork and to better understand where and how sustainable investment fits intomainstream investing.The thesis also offers theoretical and methodological guidance for future ethnographies offinance by positioning the present study with existing sociological and anthropologicalstudies and approaches. The discussion covers the political economy of sustainableinvesting with an emphasis on the links between market and society and the rise of thelarge corporation; outlines a framework for studying monetary transactions; and reflectson the nature of agency in financial markets and organisational actors there. A review ofethnographies of finance shows that studies of change within financial market practicesshould address issues of market functionality and political economy.