Microalgae, with representatives in all but one of the major algal divisions, are an important component of the biota of the worlds aquatic environments. They include phytoplankton that are almost entirely responsible for the primary production of all marine and freshwater bodies. They occur at the base of the food chains upon which the worlds fisheries industries depend, and in the numerous aquaculture projects upon which the world will increasingly come to rely on a large portion of its protein requirement. Their use for the mass production of stock feed and for direct human consumption, already being practised in many parts of the world, is likely to become of increasing importance in the future as is the exploitation of their ability to serve as a source of key metabolites in the food industry and in the synthesis of a range of other high-value products. The mass culture of microalgae under controlled conditions is also under consideration for its potential to provide an alternative source of biomass and for the production of biofuels, such as biodiesel, that does not compete for land that can be more profitably used for the production of traditional food crops. The potential of microalgae as a commercial source of another energy source, hydrogen, is also under investigation with a view to exploiting the ability of some microalgae, unique among oxygenic photosynthetic organisms, to release hydrogen gas produced from the photolytic splitting of water. The proven role of microalgae in waste-water treatment and in various environmental remediation processes as well as their potential contribution as a vital component of carbon-capture schemes will also be described. This book surveys our current understanding of those aspects of the biology of microalgae which constitute the basis of the range of practical applications now under consideration for their potential contribution to human health and well-being. The focus is largely on physiological and biochemical processes of microalgae as they are currently known, with the aim of providing some of the basic background information against which present and proposed future developments can be assessed. Many of these developments, if they are to be successful, will require collaboration of engineers, process biochemists and microbiologists as well as those trained to address economic and environmental considerations. It is hoped that this book, will provide for such workers and for the lay person, an overview of some of the relevant basic biology of the microalgae, highlighting their metabolic flexibility and their vast potential as a valuable resource that is yet to be fully realised.