The basic idea behind this book is that in a market economy there is endless variety, people die and are born, new products and processes emerge and old ones disappear etc. Some firms grow others decline. Some people get high salaries others get unemployed. Opportunities, disasters and capabilities are to a large extent random. An economy has a certain amount of resources to divide among its members. These resources may vary over time but the rate of change is fairly small. The number of persons in society may also vary but the rate of change is limited. For a society such as the one described above I was interested in deriving equilibrium distributions of various kinds and make some tests of the distributions found against data for different countries. I have studied the following types of distributions a) Income distribution b) Functional distribution of income c) Size distributions of firms. Since the above mentioned distributions are related; another main purpose of the book has been to develop a similar method for the analysis of all three distributions in order to simplify the understanding of their relations.