Poly(ethylene oxide) (PEO)-based composite polymer electrolytes have been widely studied during the last ten years. At present, systems, with room temperature conductivity up to 10(-4) ohm(-1) cm(-1) have been synthesized and tested in electrochemical devices like lithium-ion rechargeable batteries. In this paper, we will chiefly focus on the experimental work, whereas the theoretical models for ion transport will be only summarized. The systems are classified as 'blend-based' and 'mixed-phase' composite electrolytes. We call blend-based systems those systems obtained from homogeneous solutions of two components in an appropriate common solvent. Mixed-phase systems are inhomogeneous mixtures of polymer and inorganic or organic additives not dissolved in a common solvent. Effective Medium Theories (EMT) are useful for modelling the conductivity behaviour in these composite electrolytes.