The ubiquitous members of the aquaporin (AQP) family form transmembrane pores that are either exclusive for water (aquaporins) or are also permeable for other small neutral solutes such as glycerol (aquaglyceroporins). The purpose of this review is to provide an overview of our current knowledge of AQP structures and to describe the structural features that define the function of these membrane pores. The review will discuss the mechanisms governing water conduction, proton exclusion and substrate specificity, and how the pore permeability is regulated in different members of the AQP family.