While political parties provide a key channel for political representation, organised interests can also be an important mechanism. A valid question is whether a system of interest groups is capable of ensuring the representation of a variety of public and private interests. Addressing this conundrum requires data that map the essential traits of organised interest groups in a given country, including their number (density) and types (diversity). This article explores the density and composition of the Polish system of organised interests. The previously unexplored system-level approach delivers insights into the nature of the system and also provides a framework for the subsequent conceptualisation of advocacy and lobbying dynamics.A CORNERSTONE OF ANY DEMOCRATIC SOCIETY IS THE CAPACITY for its citizens to have a political voice, so that they 'can express their views, preferences, and interests to political institutions and hold public officials to account' (Fraussen & Halpin 2016, p. 476). Although political representation is achieved through voting for or joining and supporting political parties, elections are rare occurrences (every four years in Poland) and scholars question the participatory character of parties (Marsh 2006). The welldocumented debate regarding the transformation of party-political systems in Western democracies (expressed through, for example, electoral volatility and the fragmentation of party landscapes) and dissatisfaction with the representative system of government, has pushed scholars to explore the role of interest groups as mechanisms of political expression (Fraussen & Halpin 2016). 1 Against this background, it is crucial to examine the potential of the interest group system to address these democratic challenges and whether it can ensure the representation of a variety of public and private interests, namely who is mobilised, who is politically active, and who has access to the policy process (Lowery et al. 2015). The composition of the group system-its density and diversity-is an important way to judge such questions. It