In this chapter, the author presents a system for classifying time patterns as they influence economic activity. In constructing the system, he draws from ideas presented by Alfred Marshall and John Maynard Keynes. The periods Rohatinski proposes are based on changes in economic activity within each period, rather than calendar time, and range from the very short term, in which economic categories and other features of the model are unambiguous and fixed, to the very long term, in which the overall organisation and the entire pattern of the economic activity are changing.