Perovskite oxides containing transition metals are promising materials in a wide range of electronic and electrochemical applications. However, neither their work function values nor an understanding of their work function physics have been established. Here, the work function trends of a series of perovskite (ABO(3) formula) materials using density functional theory are predicted, and show that the work functions of (001)-terminated AO- and BO2-oriented surfaces can be described using concepts of electronic band filling, bond hybridization, and surface dipoles. The calculated range of AO (BO2) work functions are 1.60-3.57 eV (2.99-6.87 eV). An approximately linear correlation (R-2 between 0.77 and 0.86 is found, depending on surface termination) between work function and position of the oxygen 2p band center, which correlation enables both understanding and rapid prediction of work function trends. Furthermore, SrVO3 is identified as a stable, low work function, highly conductive material. Undoped (Ba-doped) SrVO3 has an intrinsically low AO-terminated work function of 1.86 eV (1.07 eV). These properties make SrVO3 a promising candidate material for a new electron emission cathode for application in high power microwave devices, and as a potential electron emissive material for thermionic energy conversion technologies.