This chapter reviews the state-of-the-art projection models for omnidirectional cameras and the calibration methods to compute their parameters. These models range from the expression of explicit shape of mirrors and lenses to unified and generic models. The calibration methods estimate the parameters of these models because of partially or totally known scene geometry. Various geometric features are considered for camera calibration such as points, lines circles used as input of linear estimators or nonlinear optimization schemes. Several examples of cameras and images they capture are provided together with sketches, illustrating the path of light rays entering cameras as well as the main equations using common notations to ease their understanding and comparison.