This book is about neuronal networks within the brain. With roughly 100 billion nerve cells the human brain contains about 3.2 million kilometers of 'wires' to make a million billion of connections between these nerve cells. Nerve cells exchange electrical and chemical signals through these connections and the dynamical patterns of electrical/chemical signals are the basis of all our thinking, memories, consciousness, and control of our behaviour. How do these networks develop their specific connectivity, what are the patterns of electrical activity that serve such a fundamental role in our cognitive abilities, how can it go wrong in these networks resulting in different types of brain pathologies, such as mental retardation, Alzheimer’s disease, epilepsy or schizophrenia? These questions are among the most fundamental ones in neuroscience and are addressed in this volume. The chapters in this book cover both state-of-the-art broad reviews and in-depth studies of topics selected in order to bridge different levels of neurobiological organization, from the molecular, cellular, neural network to the cognitive level. An excellent example of such a 'red thread' is given by the chapters devoted to the visual system, illustrating the advanced state of understanding of network development, plasticity and functioning of the visual system. Other chapters illustrate how scientific advances are driven by technical developments as shown for multi-electrode recording and life imaging techniques that enable the study of electrical activity simultaneously in many nerve cells.