In an era of fiscal constraint, rising public demand, and fragmented governance, the UK's approach to public spending is under unprecedented strain. At the centre of this system sits HM Treasury, long regarded as the most powerful department in British government yet questioned for its ability to deliver effective financial control in an increasingly complex and fragmented policy landscape. This book offers a bold and timely reassessment of the Treasury's role in shaping the UK's public finances over the last thirty years. Drawing on over 150 interviews with senior officials, policymakers, and frontline practitioners, it reveals how the Treasury's traditional model of top-down control has failed to adapt to the realities of modern governance over the last three decades. Through detailed case studies of prisons, special educational needs, and homelessness, the authors expose the systemic consequences of short-termism, siloed budgeting, inadequate evaluation, centralised performance budgeting, and accountability deficits that undermine both fiscal sustainability and public service outcomes.