Software is used everywhere: a modern day panacea. Still, software projects fail very often and often they fail miserably. Software engineering wisdom does not prevent these failures and associated losses, which are mainly attributed to poor management practices. This book shows that software projects can be understood as socio-technical systems, in which technical, human and social aspects related to the development processes mutually influence each other at different scales, in complex ways. Ignoring or trying to reduce complexity is one of the main reasons why these projects fail. That means, there are limits to what can be managed via simplification, or reductionism. Thus, the study of complexity stands as a paradigm shift, and it takes a major role in modern science, giving origin to the complex systems theory. This book takes a complex systems approach to software engineering and shows that complexity can be quantified and monitored, enabling project managers to take informed decisions and react timely against undesirable scenarios.