There is currently great interest in the biology of soil, stimulated by an increased concern to conserve natural resources, not only in agriculture but also in natural ecosystems. Recent developments in molecular biology have increased the possibilities of manipulating soil organisms and the processes they carry out, to improve food production and the quality of the environment. But important environmental questions associated with the application ofthis new biotechnology to soils are still to be resolved. Issues such as the desirability of using genetically-engineered microorganisms in soil can be discussed sensibly only in the light of an understanding of naturally-occurring organisms in soil. Although on a global basis food production continues to increase, man's current relationship with the soil in many parts of the world is characterised by decreasing soil fertility and increasing population pressure. Soil Science has an important role to play in the future, helping to solve these problems. Although not all areas of this large and expanding subject are covered in this book, I hope that I have been able to present the principles of soil biology in such a way that will interest the reader.