The in-work poverty risk differs within and across countries. In this chapter the authors focus on explanations of in-work poverty in and across rich countries, in particular in Europe. Combining earlier proposals they develop a model for the explanation of the incidence and structure of in-work poverty taking into account economic and institutional characteristics as well as individual and household-related factors. They provide a systematic comparative overview on previous research and single out commonly addressed topics and research gaps. Adding to the recent body of literature, they carry out a multilevel analysis based on the 2013 wave of the European Union Statistics on Income and Living Conditions (EU-SILC). The analysis shows that in-work poverty is the result of a complex interplay between individual and household earnings, household needs and the availability of transfers.