Superconductivity is an important area of modern research which has benefited enormously from experiments under high pressure conditions. The focus of this paper will be on three classes of high-temperature superconductors: (1) the new binary compound MgB2, (2) the alkali-doped fullerenes, and (3) the cuprate oxides. We will discuss results from experiment and theory which illustrate the kinds of vital information the high-pressure variable can give to help better understand these fascinating materials.