With rising consumer prices, tight labour markets, a resurgence in labour militancy, and a new post-Covid appreciation of the role of government and market regulation, the conditions for renewed union growth seem favourable. But unions have to come from far. Across the OECD, unions have lost members and power, young people increasingly stay away from unions and labour markets have thoroughly changed from what they were. How likely, then, is a new era for trade unions? This article explores four scenarios for the future of trade unions – marginalization, dualism, substitution and revitalization – each grounded in a few assumptions about the environment and behaviour of unions. Drawing on data and research from 12 OECD member states, the article explores middle range theories for each scenario and weighs arguments pro and contra its likelihood. It concludes with a set of critical propositions about the future of trade unions and research about the future.