POSEIDON models and retrieves 1D, 2D, and 3D exoplanet transmission spectra. Given a set of observed exoplanet spectra from space-based or ground-based telescopes, the code uses Bayesian techniques to infer the atmospheric properties of the planet. POSEIDON also includes disk-integrated thermal emission and reflection spectra modeling and retrievals for both secondary eclipses and directly-imaged substellar objects.