Unlike the previous chapters, our objective in this chapter is to draw cross-national comparisons rather than provide country-specifi c in-depth analysis, with the aim of assessing how cross-national differences can be explained. We introduce a systematic internationally comparative perspective on informal work and the way in which it relates to formal employment. Under what conditions is work in European societies organised in an informal form, and under which circumstances is the same type of work organised as formal employment? How can cross-national differences in relation to the extent and structures of informal work be explained? Four main areas of informal work have been examined in depth: family-based childcare; family-based eldercare; informal employment; and voluntary work (see Jensen et al. in this volume).