This paper discusses the effects of a colloidal phase on the ionic conductivity and transport mechanism in liquid and polymer electrolytes. The conductivity enhancement due to the addition of diverse ceramic colloids is attributed to the formation and existence of double layers. These colloids and associated double layers also lead to time-dependent conductivity variation in a certain temperature range. Annealing and physical aging effects on conductivity originate from the existence and displacement of the double layer surrounding each colloidal particle. It has also been suggested that one can design and develop electrolytes for specific applications by selecting three critical elements: a conducting ion, a transport medium, and a network of double layers in a colloidal or composite electrolyte. (C) 2004 Elsevier B.V. All rights reserved.