The nature and scope of the text The environment in which we live has to a large extent been determined by the activities of innumerable organisms interacting with each other and with their immediate surroundings. From the point of view of the microbiologist, it is obvious that microbial activity has a great part to play in the continuing maintenance of conditions suitable for other forms of life on this planet. There has therefore always been an awareness of the need for a good understanding of how microorganisms react in the environ ment, and this has been heightened from time to time as detrimental microbial activities become evident under certain conditions. The need for a good understanding has recently assumed a new importance as the era of microbial manipulation dawns-microbiology has always been a practical discipline, and the possibilities of beneficial modification on a global scale may be within our grasp. The growing interest in environmental microbiology can be gauged from the increase in relevant undergraduate teaching. However, one of the most serious problems confronting the student is the dearth of appropriate texts. In part this is a reflection of the plethora of potential subject matter. The study of the relationships of microorganisms with each other and with their environments-"microbial ecology" -constitutes a subject area which is far from precisely circumscribed, and each researcher or teacher has his own personal notion of which topics are appropriate.