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This thesis presents an empirical study of dendroclimatology, with the purpose of contributing to a wider understanding of the way scientists generate knowledge about climate change. Dendroclimatology is a science that produces knowledge about past climates from the analysis of tree growth. For two years, I have studied the work of a group of dendroclimatologists, joining them on fieldwork and sampling expeditions in the Scottish Highlands, observing how they generate data from tree samples to reconstruct past temperatures in Scotland and examining how they have mobilised a Scottish temperature reconstruction in a scientific debate over historical changes in climate. This thesis develops two parallel narratives about the practice of making dendroclimatological knowledge and the roles of trust and scepticism in this process. In describing how dendroclimatologists work to extract information about past climates from trees, I identify the importance of trust relationships and scepticism at each stage of their work. I conduct a symmetrical analysis of both trust and scepticism in science. In the past, scholars studying science have emphasised the critical role of either trust or scepticism in the construction of scientific knowledge, and have paid relatively little attention to examining the relationship between the two. In my study, I demonstrate that scepticism is part of the ordinary practice of dendroclimatology, and that scepticism in normal science (which I call “civil scepticism”) is fundamentally dependent (or “parasitic”) on existing trust relationships established through a variety of means. Dendroclimatologists engage in intimate interactions and mutual scrutiny of each other’s competence throughout the work they do in the field and in the laboratory, and they build upon and expand these trust relationships to create and defend climate reconstructions. I show that dendroclimatologists sustain trust relationships in part by demonstrating that they are competent sceptics (which I call “sceptical display”) and, in part by provisionally suspending their scepticism to permit agreement on what constitutes valid dendroclimatological knowledge. I also analyse how these internal practices of scepticism and agreement are influenced by sceptical challenges from actors external to the dendroclimatology community, including challenges grounded in similar trust relationships (a further instance of civil scepticism) and challenges that are not (which I call “uncivil scepticism”). I conclude that dendroclimatological knowledge is only possible as a result of contingent social negotiations over the distribution of trust and the boundaries of a trusting community.