Measurements of the Nernst effect, a transverse thermoelectric response, reveal a surprisingly large signal above Tc in high-temperature superconductors. In this talk, I discuss the theory of the Nernst signal in different parts of the cuprate phase diagram. Near the superconducting transition, we find the critical exponent for the Nernst coefficient. We apply Gaussian theory of the order parameter fluctuations further away from the transition, which enables us to explain the measured signal in overdoped samples. In the context of possible scenarios for the underdoped pseudogap regime, we consider quasiparticles with d-density wave ordering. While insufficient for explaining the Nernst effect in the cuprates, a surprising behavior is predicted for conductors with the chemical potential passing through a node in the dispersion relation.