We overview the critical steps leading to the demonstration that each ~28 kD protein monomer of the water channel (or pore)¹ pierce the lipid bilayer part of the cell membrane allowing the passage of ~ 10¹³ water molecules per second. The biophysical approach gives a functional and physical image of the water pore close to what has recently been obtained from the amino acid sequence, crystallography and other advances from the cloning era of trans-membrane water transport. Paracellular “wide” water channels of some leaky epithelia are not covered here [cf. Whittembury and Reuss, 1992; Whittembury and Hill, 2000].Sociedad Argentina de Fisiología