ABSTRACT Notable efforts have been made to apply the institutionalisation concept to legislatures, parties and party systems. In contrast, parliamentary party groups (PPGs) were either side-lined or bundled under the concept of institutionalised parliaments and parties, which implies some conceptual confusion. However, PPGs should be treated as distinct topic given their role in stabilising the parliamentary business in all modern assemblies. Encouraging research to close that gap, we introduce a framework to study the institutionalisation of PPGs for which we use illustrations from various polities. We suggest that highly institutionalised groups exhibit stable personnel, coherent legislative behaviour, structural differentiation, well-established procedural routines, and autonomy in terms of policy decision-making, leadership selection, and resources. Also, we examine PPGs as embedded institutions and provide a conceptual map for varying relationships with the institutionalisation of parliaments and parties. Finally, we point to a widely unexplored field for comparative research including investigations on democratisation.