An increasingly large percentage of power is being generated from renewable energy sources with intermittent and fluctuating outputs. Therefore there is a growing need for energy storage. With power-to-gas, excess electricity is converted into hydrogen by water electrolysis, which can be stored and, when needed, can be reconverted into electricity with fuel cells. Besides the energy vector for electricity, mobility and heat, hydrogen can be utilized as a raw material for the chemical industry or further be used for the synthesis of various hydrocarbon fuels such as methane. This article is an international review of numerous power-to-gas pilot plants that have either already been realized or are being planned. It provides information about their installed components and capacities as well as about operating experience that has been had with them. In many of the projects it was concluded that the design and sizing, control strategy and system integration of the power-to-gas plants have a great influence on their overall efficiency, reliability and economics. Topics for further research are the improvement of the efficiency, reliability, lifetime and costs of electrolyzers and fuel cells and better ways of dealing with power sources. In order to improve the overall performance, the reduction of auxiliary equipment and the continuous long-term operation of power-to-gas pilot plants will be necessary. The further development of codes and standards for permits to operate, as well as of hydrogen components and control strategies, would bring additional benefits for power-to-gas systems. It is also recommended that optimum system configurations and components be determined with regard to the available infrastructure and the type of application involved. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.