The Labour party has been largely defined by the battle of ideas between its left and right sections. This book chronicles those battles, highlighting the key ideas and people behind them since Attlee. It focuses on the totemic moments in the party, but pays particular attention to the Corbyn era. It argues that to understand Corbynism it is important to understand the previous battles over ideas in the party. The final chapter analyses how Corbynism has reacted to Brexit, the biggest issue to face UK politics since WWII, and how this issue has re-opened the left and right battle.