The problem of the stabilization of moduli is discussed within the context of compactified strongly coupled heterotic string theory. It is shown that all geometric, vector bundle, and five-brane moduli are completely fixed, within a phenomenologically acceptable range, by nonperturbative physics. This result requires, in addition to the full space of moduli, nonvanishing Neveu-Schwarz flux, gaugino condensation with threshold corrections, and the explicit form of the Pfaffians in string instanton superpotentials. The stable vacuum presented here has a negative cosmological constant. The possibility of “lifting” this to a metastable vacuum with positive cosmological constant is briefly discussed.