Whether the redox state, quantified as oxygen fugacity, recorded in a planetary basalt is an accurate representation of the redox state of the planetary interior from which it was derived through partial melting, ascent, eruption and emplacement is a fundamental question in planetary geology. In the absence of mantle xenoliths in samples from the Moon, Mars and differentiated asteroids, the basalt-mantle source relationship must be extrapolated from what is known about the Earth in order to probe the redox state of these planetary interiors. A review of current knowledge regarding the basalt-mantle source relationship for the Earth provides insights into the advantages and pitfalls of determining mantle redox state. The range of currently available oxybarometers, including thermodynamic models based on ferrous-ferric mineral equilibria and multivalent cation analysis are surveyed and their limitations presented. The result is a basis for the informed interpretation of the oxygen fugacity of planetary basalts, and new insights into the role of C-H-O volatiles in the terrestrial planets.