The book begins with an overview of the cyanobacteria - or blue-green algae, for those who are not specialists - then looks at their diversity in the geological record and goes on to describe their ecology in present environments where they play important roles. Why is one of the key groups of organisms in the Precambrian still one of the most important groups of phototrophs today? The importance of ecological information for rational management and exploitation of these organisms for commercial and other practical purposes is also assessed. Accounts are provided of nuisances as well as the ecology of the commercially successful Spirulina and the role of cyanobacteria in ecosystem recovery from oil pollution.