Which institutions may be important in terms of trade union density and how significant they are? Although the status of trade unions may be very different among states, unions are still a very meaningful component of labour markets. In this paper, we contribute to the debate about the institutions that may affect the outcome of trade unions in different legal systems. Firstly, we draw on the theoretical underpinnings of trade union activity and density. Then, we conduct an empirical analysis of the relationships between trade union density in a particular country, country’s legal origins and government’s ideology. In this way, the paper enriches an underexploited niche in institutional research devoted to labour market issues.