This special issue is devoted to discussion of probability-based survey panels that collect data either solely or partly through online questionnaires. Panels of this kind have been around for a long time, though they have been few in number, but recent years have seen several new panels start up in Europe. This has led to renewed interest in the methodology of such panels and also to deeper questioning of the role of these panels. On one hand, the probability-based panels are to some extent competing against cheaper non-probability access panels. On the other hand, the probability-based panels are increasingly being seen as possible alternatives to more expensive probability-based survey methods. In both cases, clients and data users want to better understand the relative advantages and disadvantages of the probability-based panels.