This contribution critically reviews existing data and other recent sources to provide an overview of the dimensions and dynamics of contemporary irregular migration. First it considers the utility and uses of irregular migrant statistics, considering their inherent inaccuracy, their inability to distinguish different types of irregular migrant, the way that data is collected, and the purposes to which it is put. The second section reviews the conceptual and practical challenges associated with counting irregular migrants. Conceptual challenges include: differentiating stocks from flows, addressing the variety of routes into irregularity, distinguishing migrant smuggling from human trafficking, separating asylum from aggregate statistics, and acknowledging that migrants’ legal status can change quickly. Practical challenges associated with a series of direct and indirect methods for collecting statistics (including national censuses) are reviewed, and the difficulty for researchers to access official statistics highlighted. The third section presents estimates of the scale and scope of irregular migration, and data from regularisation programmes in various high-income countries. The fourth section reviews explanations for irregular migration, distinguishing macro-level explanations that focus on structural causes from meso-level explanations concerned with the role of policies and intermediaries, and also notes the shortage of micro-level explanations concerned with individual and family decision-making. The final section considers the consequences of irregular migration, with a particular focus on security. The review suggests that while irregular migration can undermine state sovereignty, the implications of irregular migration for the human security of those involved should not be underestimated. The conclusion considers some implications for future research