Causality: Cause and effect. In classical physics, an effect cannot occur before its cause. In Einstein's theory of special relativity, causality means that an effect cannot occur from a cause that is not in the back (past) light cone of that event. The books cover the useful physical relations inferred by unitarity and causality. A famous example is the Kramers-Kronig relation for the refractive index of a gas or dilute medium. For example, in Chapter 3 generalises the Kramers-Kronig relation to relativistic medium, such as CMB (photon gas). These relations are extensively also used in particle and nuclear physics. Especially useful are the so-called “sum rules”, such as the Gerasimov-Drell-Hearn (GDH) or the Baldin sum rule. The author notes that the first edition is too brief. In his teaching practice, he sees that is is challenging to use as a standalone text. He intends to improve the explanations of many topics that students found particularly challenging. The additional material will make the book more timely, self-contained, and logically complete. Key Features: Modern book on a timely subject Classroom friendly and built on newly developed courses taught at a major Physics Department in Germany Advanced material written in simple non-technical language Includes pedagogical examples and discussions of the current forefront research