A cosmological model is a description of the universe in terms of its spacetime configuration. Assuming that matter, overall, is distributed evenly and the same in all directions, there are three basic possibilities. (1) A closed universe is positively curved like the surface of a ball and is finite in extent; such a universe grows to a certain size before collapsing again. (2) An open universe is negatively curved like a saddle and is infinite in extent. (3) A flat universe has zero curvature and is infinite in extent. The factor determining which kind of spacetime we actually live in is the average density of matter in the Universe. Depending on whether the average density is greater than, less than or equal to the so-called critical density, the cosmos is closed, open, or flat, respectively. Current observations suggest a value tantalizingly close to that needed for spacetime to be flat.