This special issue of Chemical Reviews covers the electrochemical storage and generation of energy in batteries and fuel cells. This area is gaining tremendous importance for powering high technology devices and for enabling a greener and less energy-intensive transportation industry. Whether the demand is from a cell phone, a computer, or an iPOD, consumers are demanding a longer life in a smaller package and at a lower cost with minimal if any wired connection. The consumer generally does not care whether the power source is a battery, a fuel cell, or something else, as long as it works. In the area of greener transportation, there has been a surge of interest in vehicles that are electrically powered, either totally, as planned for the green Beijing Olympic Games, or partially, as in hybrid electric vehicles. The present generation of such vehicles uses a combination of an internal combustion engine and a battery, today nickel metal hydride, as in the Toyota Prius, and tomorrow lithium; a future generation is likely to be a hybrid of a fuel cell and a battery. Both batteries and fuel cells utilize controlled chemical reactions in which the desired process occurs electrochemically and all other reactions including corrosion are hopefully absent or severely kinetically suppressed. This desired selectivity demands careful selection of the chemical components including their morphology and structure. Nanosize is not necessarily good, and in present commercial lithium batteries, particle sizes are intentionally large. All batteries and fuel cells contain an electropositive electrode (the anode or fuel) and an electronegative electrode (the cathode or oxidant) between which resides the electrolyte. To ensure that the anode and cathode do not contact each other and short out the cell, a separator is placed between the two electrodes. Most of these critical components are discussed in this thematic issue. The issue starts with a general introduction by Brodd and Winter to batteries and fuel cells and the associated electrochemistry. It then continues first with several papers discussing batteries and then with papers discussing fuel cells.