This book offers a unified view of emergence and complexity. It also looks at theory-change as the evolution of the complexity of theories. The author makes use of concrete historical cases analyzed in the tradition of the philosophy of science to test the theses of this work. The first chapter deals with the historical background on emergence. It discusses several contemporary works on the subject, their different points of view, and their contributions to the present investigation. Next, the author details what is now known as complexity science. He covers methodologies of this branch of knowledge, taking into account the amount of information used to describe the system under study. After this foundation, coverage offers a novel quantitative definition of an emergent property. This is based on another notion introduced in this work: the parametric-model complexity. The author discusses several concepts that involve the importance of small variations of parameters that regulate the system and the universal characteristics that these small variations generate. He then studies how this new notion can shed light on some aspects of scientific practice as the criteria for the choice between rival theories and the relationship between the emergence of new technologies and the risk in the appearance of anomalies.