Large Natural Cherenkov Detectors: Water and Ice

In this review we first address two questions: 1. Why do we need kilometer-scale muon and neutrino detectors? 2. What do we learn from the operating Baikal and AMANDA detectors about the construction of kilometer-scale detectors? I will subsequently discuss the challenges for building the next-generation detectors. The main message is that these are different, in fact less ominous, than for commissioning the present, relatively small, detectors which must reconstruct events far outside their instrumented volume in order to achieve large effective telescope area.