The goal of this book, is to recall to the systems community an important challenge to be dealt with in the immediate future: the study and characterization of general features of what is commonly qualified as `emergence', chiefly in complex systems such as biological and cognitive ones. Such a topic was a fundamental one at the very beginning of the systemic movement, and to it the founding fathers, such as Von Bertalanffy, Ashby and Von Foerster, devoted most efforts. In more recent times, however, the interests shifted towards an empirical study of systemic properties characterizing human organizations, and the subject of emergence was partly abandoned. Notwithstanding, the understanding of what is emergence, and of the circumstances which allow for its occurrence within a complex system, is of crucial importance for systemics. Namely all systemic properties - the ones which allow a system to behave as a whole and not as an aggregate of constituents - are just emergent properties.