We review the characteristics and the properties of various types of gel-type, ionically conducting membranes. We have mainly investigated two classes of membranes, one characterized by lithium ion transport and the other characterized by proton conductivity. We show that the former membranes are suitable to be used as separators in advanced lithium ion plastic batteries and that the latter show good promises to be considered as alternative, new separators in polymer electrolyte fuel cells. (C) 2003 Elsevier B.V. All rights reserved.