The noblest pleasure is the joy of understanding.-Leonardo da Vinci
IntroductionBeginning with the Rindler horizons which appear for accelerated observers in Minkowski space without gravity, one comes quickly to acknowledge the presence of various types of horizons when gravity is introduced in spacetime: first one encounters black hole horizons and cosmological horizons. Then, studying classical and semiclassical black holes one is faced with inner, outer, Cauchy, and extremal horizons. The early literature on black holes and the works which developed black hole thermodynamics in the seventies had plenty to do with discussing stationary black holes and event horizons (e.g., [23,74,75]). Dynamical situations such as gravitational collapse, black hole evaporation due to Hawking radiation, and black holes interacting with non-trivial environments and exchanging mass-energy require that the concept of event horizon be generalized to some other construct with which it is possible to work. Conceivable dynamical situations include black holes accreting or expelling gravitating (i.e., non-test) matter; examples are Vaidya spacetimes, black holes immersed in a cosmological "background" other than de Sitter space, black holes emitting (and possibly also absorbing) Hawking radiation (which becomes significant in the last evolutionary stages with backreaction playing an important role), or black holes with variable mass because of other physical processes. If a black hole is placed in a non-trivial environment, its mass-energy should be also the internal energy which we need to account for in the first law of thermodynamics. This mass-energy must be defined carefully; usually it is identified with some quasi-local energy construct which is related to the notion of horizon. In these lectures we will use the Misner-Sharp-Hernandez mass in spherical symmetry and its generalization, the Hawking-Hayward quasi-local energy in the absence of spherical symmetry.Intuitively, an horizon is "a frontier between things observable and things unobservable" [64]. Inequivalent notions of black hole horizon abound in the technical literature and the terminology used features event,