The series on 'Charged and Reactive Polymers' was set forth in two volumes concern­ ing the fundamentals and applications of polyelectrolytes. A follow-up on 'Charged Gels and Membranes' would therefore seem appropriate, necessitating, however, some explanation for non-specialists. Theories of the most dilute gels originate in that of concentrated polyelectrolytes: the methods and problems are similar in structural, spectroscopic or thermodynamic properties. The borderline can be situated in dialysis conducted with a 'bag' imper­ meable to polyelectrolytes but not to small ions, solutes and water. One may recall Donnan's use of such a system to experiment and discover his famous law of unequal distribution of ions of different charge inside and out. Remark­ ably so, it is the difference in scale which characterizes the difference between poly­ electrolyte solutions and gels and membranes: the colloidal solution of macro­ molecules is heterogeneous only on the microscopic level, whereas the gel-solution system is a macroscopically heterogeneous one. A gel is formed when weak or strong cohesive forces counterbalance the dispersing ones (usually by crosslinking) without inhibiting the penetration of solvent and of small solutes into the polymeric network. The solvophile macromolecules cannot invade the total volume of liquid. As a result of phase-segregation excess solution and gel coexist and interact. The macroscopic swelling depends on gel cross-linking as well as on ionic concentration and type and ion-selectivities are observed.