The problem of the 'secondary hydration force ' that appeared in early measurements of double layer interaction was introduced on the first page. In 1997 I started calculations of double layer properties using ion-ion potentials of mean force in water calculated in simulations. The method was formally established as an application of the McMillan-Mayer theory to nonuniform fluids during a visit to Tel Aviv University in 1999. With a rigorous background and physically interesting results I believe that the method will become a useful tool for study of more complicated biological problems. In the example shown in the figure, the surface charge is 75 Å2 per unit charge, electrolyte concentration is 0.1 M NaCl and we show pressure between flat planar surfaces. Two curves that include short-range interaction between ions are very much reminiscent of surface force measurements. In the example with adsorption, 50% of the surface charge is compensated by adsorbed counterions. The figure is taken from the article by R. Kjellander, A. Lyubartsev and S. Marcelja, to be published in J. Chem. Phys.