Let us start with the basics: what is a “system” and what does it mean to characterize a system as “generic” and “quantum”? What is “information” and how can it be “distributed” between systems? What, finally, does it mean to say that some physical process is a “computation”? This book addresses these questions, and explores the consequences of answering them while making as few assumptions as possible. It shows how answering these simple questions leads to a deep theory with applications throughout physics, and beyond physics into the life and social sciences. This theory is universal, applying to all physical systems at all scales of organization or complexity. It is also scale-free, describing systems at all scales with the same formalism and in the same way. It achieves this by formalizing all physical interactions as information exchange.