This thesis examines the place of idealism and historicism, interpreted as one complex tradition, in twentieth century British historical thought. It contributes to the intellectual history of modern British historiography and philosophy of history by arguing that idealism-historicism was pivotal in their development, even if not in a straightforward or solely positive fashion. The received wisdom, holding idealism-historicism to be a relic of the nineteenth and early twentieth century, is thereby decidedly challenged. Idealism-historicism, it is shown, broke the boundaries of professional disciplines, ideological stances and generations. British intellectuals as diverse as R.G. Collingwood, Peter Laslett, Hugh-Trevor Roper, G.R. Elton and Quentin Skinner all committed themselves to the central theme of idealism-historicism: agency. This theme is distinctive to the British context, and so enables an historical and conceptual appreciation of that specificity. Agency entails a view of the past as constituted by human individuals, characterized by their capacities of freedom and rationality, necessarily and always exercised in particular socio-historical contexts; and it entails a view of history as the discipline that studies the various and changing embodiments of agency. Even though all scholars investigated subscribed to these views, the thesis further demonstrates how other beliefs—intellectual, personal and ideological—and different contexts, complicated their relationship to idealism-historicism. The Second World War was, above all else, the factor crooking the trajectory of this tradition. Whereas intellectuals from the pre-war and war generation advocated a thick concept of agency, including irrationality, emotion and public engagement fostering the political extension of agency, the post-war generation, socialized in a context of social security and political stability, and dismissive of intellectual involvement in public affairs, reduced the concept of agency to free, rational and social activity, to be studied and used primarily within the confines of professional human scientific scholarship.