The evaluation and interpretation of redox (i.e, fO2) “messages” carried by silicate liquids and their crystallization products, both terrestrial and planetary, has been a major theme and active field of petrologic research for the last 40 years. Chromium is a minor, yet consequential, multi-valent element that is present at dilute concentrations in nearly all basaltic magmas. Recent work has shown that synchrotron μ-XANES measurements of Cr valence in early liquidus olivine phenocrysts can be exploited to yield quantitative information on the prevailing fO2 conditions of primitive mantle-derived basaltic liquids. Spatially resolved measurements of Cr valence in olivine are also unique in that they can be placed into the context the growth history of the phenocryst, thus potentially revealing a wealth of information about temporal variations in the oxidation state of the magmatic system as a function of crystallization and the compositional evolution of the residual liquids. In this way, olivine phenocrysts may serve as high fidelity recorders of magmatic fO2 that can provide new insights into the connection between the oxidation state of basaltic magmas and their mantle sources. Decoding the Cr2+/ΣCr of natural olivine phenocrysts and exploiting this information as a redox “chronometer” is a challenging petrologic problem that requires a multifaceted experimental, analytical, and thermodynamic modeling approach to understand how Cr2+/ΣCr values are dictated by temperature, fO2 and melt chemistry.