What do we really mean when we say a political party has changed? And exactly what is it that drives that change? Political scientists working in the comparative tradition have come up with a general explanation that revolves around the role of election defeats and loss of office, and around changes of leader and factions. But how well does that explanation cope when subjected to a historically-grounded and therefore robust examination? This book tries to answer that question by subjecting the common wisdom to a real-world, over-time test using one of the world's oldest and most successful political parties as an in-depth case study. What do the periods spent in both opposition and government by the British Conservatives since 1945 tell us about what drives parties to change their sales-force, the way they organize, and the policies they come up with? Using internal papers, memos and minutes of meetings from party archives, along with historical and contemporary accounts, memoirs and interviews, this book maps the extent of change and then explores what may have driven it. The conventional wisdom, it turns out, is not necessarily wrong but incomplete, requiring both qualification and supplementation. This approachably-written book suggests when, how, and why. Along the way, it provides a fresh and comprehensive account of the Conservative Party that should appeal equally to those interested in political history and those interested in political science.