The growing importance of portable and transportable power sources is emphasised. This is to some extent associated with the growth in the use of cordless appliances of all types, but the range of applications storage batteries, the reliability of which has been well established, is also steadily expanding. Claims have been made that there are now 95 distinct uses for secondary cells and batteries. The use of portable power sources would, however, be greatly extended by the development of systems with higher energy and power densities, particularly in the field of electric traction.The basic principles of these electrochemical power sources are outlined, and the manufacturing processes and performance of the main types briefly described. Some details are given of newcomers, capable of much higher outputs than conventional batteries; in particular, sodium-sulphur, lithium-chlorine and zinc-air cells. The commercial viability of these has yet to be determined. Fuel cells are described in some detail and indications given of outstanding problems.Finally, the characteristics of the various power sources are summarised, and the relationships between the power and energy densities are illustrated graphically. While significant advances have been made in the performance of conventional types during the past 20 years or so, these appear to be approaching the limits of their capabilities under present régimes of use. Outstanding problems make it unlikely any of the new systems described will come into large-scale commercial use within the next 10 years.