Abstract: In the Cardassian model, the current acceleration of the universe is explained by modifications to the Friedmann equation (Einstein's equation as applied to a homogeneous isotropic universe). The universe is flat, matter dominated, and accelerating, and contains no cosmological constant or other vacuum component. The energy density required to close the universe is much smaller than in a standard cosmology, so that matter can be sufficient to provide a flat geometry. The new term required may arise, e.g., as a consequence of our observable universe living as a 3-dimensional brane in a higher dimensional universe. The Cardassian model survives several observational tests, including the cosmic background radiation, the age of the universe, the cluster baryon fraction, and structure formation. Future supernova data can be used to differentiate between models.