The thermodynamic stability and electronic structure of 40 surfaces of lithium peroxide (Li2O2) and lithium oxide (Li2O) were characterized using first-principles calculations. As these compounds constitute potential discharge products in Li-oxygen batteries, their surface properties are expected to play a key role in understanding electrochemical behavior in these systems. Stable surfaces were identified by comparing 23 distinct Li2O2 surfaces and 17 unique Li2O surfaces; crystallite areal fractions were determined through application of the Wulff construction. Accounting for the oxygen overbinding error in density functional theory results in the identification of several new Li2O2 oxygen-rich {0001} and {1 (1) over bar 00} terminations that are more stable than those previously reported. Although oxygen-rich facets predominate in Li2O2, in Li2O stoichiometric surfaces are preferred, consistent with prior studies. Surprisingly, surface-state analyses reveal that the stable surfaces of Li2O2 are half-metallic, despite the fact that Li2O2 is a bulk insulator. Surface oxygens in these facets are ferromagnetic with magnetic moments ranging from 0.2 to 0.5 mu(B), In contrast, the stable surfaces of Li2O are insulating and nonmagnetic. The distinct surface properties of these compounds may explain observations of electrochemical reversibility for systems in which Li2O2 is the discharge product and the irreversibility of systems that discharge to Li2O. Moreover, the presence of conductive surface pathways in Li2O2 could offset capacity limitations expected to arise from limited electron transport through the bulk.