Throughout the last century, stream ecologists tried to answer the question: how do benthic invertebrates cope with the flows prevailing in streams? Whereas the pioneers frequently sought answers using imagination and speculation in a hefty debate, subsequent research on flow adaptations of stream invertebrates relied increasingly on the transfer of concepts (from fluid mechanics to stream ecology) and technological innovations. Correspondingly, views about flow adaptations of stream invertebrates changed considerably over the last century. However, stream ecologists are still far from understanding how stream invertebrates are adapted to the many different flow conditions they face during their life, because the near-bottom flows they experience are extremely complex and create so diverse constraints that adaptation to all of them is physically impossible.This instance shows how ignorant we are of the physical factors in the environment which ultimately shape the organisms, and how difficult it is to understand the utility of a structure without knowing the requirements for which it is produced SUNDER LAL HORA, 19301. An Obvious QuestionCollecting benthic invertebrates in lowland streams, one immediately notices that the fauna in swift-flowing riffles differs from that in pools. In lowland streams, the riffle has a stony and the pool has a sandy bottom, so one cannot decide if these faunal differences relate to differences in velocity or bottom substrate. Repeating the exercise in mountain streams, where both riffles and pools have a stony bottom, one concludes that current causes the faunal differences. Thus, not surprisingly, a major question of stream ecologists was and still is: how do the invertebrates cope with swift currents prevailing in stream riffles? Here, I will review literature from different periods of the last century to illustrate the changing answers stream ecologists provided to this question, focusing on (1) drag (the major concern over one century) and deliberately omitting flow adaptations related to filter-feeding techniques; (2) the increasing shift from intuitive to more informed views of the relevant fluid mechanics; and (3) the controversy between those who saw the peculiar features of stream invertebrates as flow adaptations and those who saw them as prerequisites to colonize swift-flowing streams.