The Politics of Co-Opposition reveals how a completely new form of political engagement in the British Isles – the 2021-24 Co-operation Agreement between Plaid Cymru and Welsh Labour – created history and provided the major part of the Welsh Government’s policy programme for over three years. John Osmond, who was involved in negotiating the Agreement as Special Adviser to Plaid Cymru leader Adam Price, provides an insider’s account of the political background to the Agreement – including a fascinating week-by-week diary of Price’s first 100 days – how it was finalised and the compromises that were made to achieve it. Essential reading for politicians, political journalists, students and political scientists globally, the Co-operation Agreement, termed by academics as ‘Contract Parliamentarianism’, drew on non-coalition precedents in Sweden, New Zealand and Malaysia. It resulted in significant measures being introduced across 46 policy areas such as: free school meals for all primary school pupils, expanding free childcare to all two-year-olds, action on the second homes crisis blighting rural Wales, and reforming the Senedd (the Welsh Parliament) including a 60% increase in Senedd Members – from 60 to 96 – and a fully proportional electoral system from 2026. Brought to a premature end in May 2024 by Plaid Cymru as the result of the controversial internal election of the new Labour leader which resulted in the implosion of his short-lived government, The Politics of Co-Opposition is a fascinating and candid account of how innovative politicians co-operated on key mutually-agreed policies while maintaining their positions as government and opposition.