Aquaporins comprise a family of water-transforming membrane proteins. All aquaporins are efficient water transporters, while sustaining strict selectivity, even against protons, thereby maintaining the proton gradient across the cell membrane. Recently solved structures of these membrane channels have helped us to understand this remarkable property. The structure of the Escherichia coli glycerol facilitator GlpF at 2.2 Å resolution has enabled the refinement of a low-resolution human aquaporin-1 structure. This latter structure has recently been confirmed by the 2.2 Å structure of bovine aquaporin-1. Further insights, particularly with respect to the dynamics of water permeation and the filter mechanism, have come from recent molecular dynamics simulations.