As many of the authors in this book note, the energy available in the hydrogen currently produced around the world is relatively small when compared to the global primary energy use. Such comparisons however yield a poor estimate of hydrogen's importance as we undergo the necessary transition towards low carbon intensity energy generation systems. There are many possible scenarios for future global supply of energy. Production of hydrogen is of vital importance if we wish to secure reliable supplies of low carbon energy. This book is about the methods which can be used to produce the hydrogen that is needed. The methods presented range from those which seek to further develop existing technologies through refinement of the catalysts used, to new methods relying on direct use of sunlight or micro-organisms. The material presented ranges from topic reviews to experimental studies, many of which are at the forefront of their respective areas.