The study of irregular migration as a specific social phenomenon took off during the 70s in the US. Since then, the academic interest has continually grown and spread, first to Europe and, in the last years, to other regions worldwide. This interest can certainly be related to the increasing attention paid to the study of migrations more in general (Castles & Miller, 1993). The trend can be linked to those broad and complex social and economic changes, often subsumed under the concept of globalization. The specific focus on irregular migration, though gaining momentum throughout the 1980s, reached preeminent attention in the 1990s. On both sides of the Atlantic, the explosion of the so-called "migration crisis" (Zolberg & Benda, 2001) and the emergence of irregular migration as a widespread social fact raised the attention of public opinion and academics alike. Moreover, in recent years, what seemed at first to be an issue concerning only the high-income regions of the planet, now involves also medium and low-income ones, making irregular migration a truly global structural phenomenon (Cvajner & Sciortino, 2010a; Düvell, 2006). Accordingly, after a lapse of two decades, a topic that for a long time had been relatively marginal (Anderson & Ruhs, 2010; Bloch & Chimienti, 2011) became the object of numerous studies and of a consistent and diversified literature. Given the complexity of the phenomenon, its multiple dimensions, and levels of social interaction, its study has inevitably taken a multidisciplinary path. The literature has rapidly expanded in many directions and, today, irregular migration constitutes an important subfield within migration studies. This chapter will present a general overview of the main directions and developments that the research on irregular migration has taken. Even if some scholars have lamented a limited cumulative effort, many studies are available and it is now possible to refer to them as a solid starting point for analysis. To avoid getting trapped in difficult and sometimes redundant disciplinary distinctions, this overview will focus on the key issues that have been researched from different perspectives. In this respect, it seems possible to identify six main general thematic fields.