The phenomenon of growth is virtually universal; almost all the objects of human study grow: [eg] crystals, molecules, cells, plants, animals, children, personalities, knowledge, ideas, cities, cultures, organizations, nations, wealth and economic systems. It does not follow from the mere universality of the growth phenomenon that there must be a single unified theory of growth which will cover everything from the growth of a crystal to the growth of an empire. Nevertheless all growth phenomena have something in common, and what is more important, the classification of forms of growth and hence of theories of growth seems to cut across most of the conventional boundaries of the sciences. In addition there are a great many problems which are common to many apparently diverse growth phenomena.
Several types of growth (or decline) may be usefully distinguished: simple growth, involving the increase of a single quantity; population growth, in which the growing quantity is analysed into an age distribution; structural growth, in which the growing aggregate consists of a complex structure of interrelated parts and in which growth involves change in the relation of the parts; merging into structural change or development, in which it is not the size which is growing, but the complexity or the systemic property.