A two-dimensional (2D) imaging technique, planar optode (PO) technique, is developed to investigate biogeochemical processes at a high resolution in heterogeneous sediments and soils, especially at the sediment-water interface. Compositional changes with depth in sediments are usually considered to be relatively steady. However, the activities of benthic organisms can generate significant heterogeneity and complex 3D transport-reaction patterns over millimeter to meter scales in the surficial sediments. Spatial and temporal quantification of the variables such as O2 in the bioturbated sediments promoted the development of the planar optode. Different planar optode techniques including composite planar optode for multi-analytes and the combination of planar optode with diffusive gradients in thin films (DGT) have been developed and used to study the 2D solutes distributions and/or dynamics of O2, pH, partial pressure of CO2 (pCO2), temperature, exoenzymes, and metal/metalloid ions in sediments and soils. New findings observed in laboratory-based microcosm experiments and field-based in situ studies using planar optode techniques, have significantly improved our understanding of heterogeneous biogeochemical reactions and processes. In this article we provide a critical review on the: 1) research progress using planar optode techniques; 2) principles, configurations, and devices used for planar optode systems; 3) characteristics and interferences associated with planar optode measurements; and 4) applications of planar optode in the environment. We have suggested the barriers, advantages, and research needs for the use of planar optodes.