The overall aim of the MIrreM project is to address the uncertainty and contested nature of evidence on irregular migration by fostering a shared understanding of basic features of the phenomenon, and in particular, its quantitative dimension, including the scope and importance of different pathways into and out of irregularity. Developing a clear conceptual framework that transparently describes, justifies and communicates how irregular migration can be understood is a key element of such an undertaking and the focus of this paper. Specifically, this paper provides a conceptual framework for the identification of the population in an irregular situation for the purpose estimating and quantifying this population. In so doing, the paper considers both the irregular migrant population, as well as other migrants with an unstable legal status. Distinguishing between three overarching status categories, it classifies the population of irregular migrants and related categories into different ‘classes’. A typology of pathways into and out of irregularity, that is, of ways how migrants become or cease to be in an irregular situation is a key element of the classification system. The MIrreM classification of migrant irregularity builds on the rich body of work on migrant irregularity. Specifically, we draw on the model of migrant irregularity developed by the Clandestino project (2007-2009). In the context of the MIrreM project we propose several refinements to this classification systems, specifically also in relation to the categories of migrants considered and the ‘classes’ of migrants distinguished within the various categories. This paper is thematically linked to and complemented by the MIrreM ‘taxonomy of migrant irregularity’.