Complex Systems Science is a cross-disciplinary endeavor, linking researchers in physics, computer science, and mathematics with applications in diverse areas such as information and communication technologies, epidemiology, social sciences, economics, and many more. The discovery of the laws governing the structure and dynamics of complex systems is one of the greatest challenges of modern science. These systems are characterized by the presence of different levels of self-organization, each determining the behavior of the next. One of the most important properties of complex systems is the impossibility to predict their overall behavior from the knowledge of the properties of their components. Understanding and predicting the behavior of these systems is of great importance in contexts as diverse as socio-economic, biological or technological fields. The science of complex systems not only provides a way to answer open questions, but also promotes a new language and concepts that will allow the tackling of many current and pressing problems.