The skyline operator has been studied in database research for multi-criteria decision making. Until now the focus has been on the efficiency or accuracy of single queries. In practice, however, users are increasingly confronted with unknown data collections, where precise query formulation proves difficult. Instead, users explore the data in a sequence of incrementally changing queries to the data to match their understanding of the data and task. In this paper, the authors study the skyline operator as a tool in such exploratory querying both analytically and empirically.