In order to find the three-space coordinates of a scene from a pair of images it is necessary to obtain a "camera model" for each image. If the geometry and physics of the camera are complicated, it may be more convenient to develop the camera model from phenomenological considerations rather than from exact geometric and physical considerations. Thus, some properties of a general camera model are investigated. A general camera model is defined as a transformation from three-space onto two-space such that the pre-image of any point in two-space is a straight line. Therefore, an arbitrary transformation from three-space onto two-space is not a camera model. The most general camera model which maps any straight line in three-space to a straight line in two-space is developed. Finally, the general properties of a pair of images which possess an "epipolar geometry" are examined, and an example of a camera model which yields "epipolar curves" (as opposed to lines) is given.