Book excerpt: Topology is the study of properties that remain unchanged under bending, twisting, and stretching of a system. Long important in other areas of physics, topology has recently become a vital tool in optics and photonics as well, leading to a broad range of unexpected phenomena such as knotted and linked optical vortices and topologically protected states that are immune to environmental disruption. Topology in Optics: Tying Light in Knots (Second Edition) covers the background needed to follow these developments and provides a brief survey of topological effects in optics. Assuming only a background in physics at the advanced undergraduate level, it requires no prior familiarity with topology. Revised and expanded with two new chapters. Topological Photonics and Optical Knots and Links, this will be an invaluable reference for undergraduate and graduate students as well as researchers and engineers in optics and related areas. Book jacket.