Introduction Greens in the United Kingdom (UK) and Ireland have a long history of participation in electoral politics, spanning over four decades. They have experienced highs and lows but have maintained a permanent presence in party politics. There are a number of Green parties operating in the UK and Ireland – the Green Party of England and Wales (GPEW), the Scottish Green Party, the Green Party in Ireland and the Greens of Northern Ireland. 1 Each has its own unique story, in the context of UK and Irish politics, from long-term but limited electoral impact (the Greens in Britain) to involvement in coalition government (the Irish Green Party). However, Greens across the islands of the UK and Ireland are characterised by durability and persistence, despite the relatively inhospitable political systems in which they operate. This chapter outlines the major developments in the evolution of Green party politics in the UK and Ireland. The discussion aims to document the electoral journey of the Greens, to investigate their ideological and policy character and to examine their organisational form. The radicalism of Green ideas has largely remained intact, but this can be interpreted as a sign of weakness, with Greens remaining outsiders in an unfavourable political environment.