What is the toughest recognition problem in biology? Arguably, it is that faced by antifreeze or thermal hysteresis proteins. These proteins provide a broad range of organisms with protection against freezing damage by depressing, in a noncolligative manner, the freezing point of water. They do this by binding to nascent ice nuclei and inhibiting their growth (1). In other words, antifreeze proteins (AFPs) must distinguish one phase of water, ice, from another phase, liquid. Moreover, the latter is present in great excess at 55 M. There are no chemical differences to key off, just the subtle structural differences, still poorly characterized, that exist between the surface of ice nuclei and liquid water. In addition to the intrinsic interest of the ice recognition problem, AFPs have application to cryopreservation of tissues and organs (2) and to the food industry (3). The work of Davies and colleagues (4) in PNAS represents a significant step forward in understanding the detailed mechanism of AFP action (i.e., a first look, if you will, at an AFP in the act of discriminating between different structural states of water). To put this work into perspective, it is useful to have some background on the study of AFPs, which has a number of paradoxical elements.