Positronium forms in various insulating solids. Observation of the energy of the positronium emitted from the surface carries information on the relevant energies inside the material. Analysis of the positronium peak in ACAR spectra gives information on the Bloch modulation of the Ps wave function, the Ps-phonon interactions, the size and anisotropy of the Ps localization site, Ps self-trapping, etc. Magnetic quenching is applied to investigate the positronium pickoff annihilation rate, contact density of the Ps wave function, and positron spin polarization.