In particle physics, avour or avor refers to the species of an elementary particle. The Standard Model counts six avours of quarks and six avours of leptons. They are conventionally parameterized with avour quantum numbers that are assigned to all subatomic particles, including composite ones. For hadrons, these quantum numbers depend on the numbers of constituent quarks of each particular avour. In atomic physics the principal quantum number of an electron specifies the electron shell in which it resides, which determines the energy level of the whole atom. In an analogous way, the five flavor quantum numbers of a quark specify which of six flavors (u, d, s, c, b, t) it has, and when these quarks are combined, this results in different types of baryons and mesons with different masses, electric charges and decay modes. This book gives an overview of the category of "flavor" in particle physics."