Is the universe fine-tuned for complexity, life, or something else? This comprehensive overview of fine-tuning arguments in physics, with contributions from leading researchers in their fields, sheds light on this often used but seldom understood topic. Each chapter reviews a specific subject in modern physics, such as dark energy, inflation, or solar system formation, and discusses whether any parameters in our current theories appear to be fine-tuned and, if so, to what degree. Connections and differences between these fine-tuning arguments are made clear, and detailed mathematical derivations of various fine-tuned parameters are given. This accessible yet precise introduction to fine-tuning in physics will aid students and researchers across astrophysics, atomic and particle physics and cosmology, as well as all those working at the intersections of physics and philosophy.