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This chapter describes the use of ultrathin oxide overlayers to modify and control the properties of electrocatalysts. Of particular interest are oxide-encapsulated electrocatalysts (OECs), for which the oxide overlayers are permeable to electroactive species such that electrochemical reactions occur at the buried interface between the overlayer and active electrocatalyst component. Such OECs offer several advantages over conventional electrocatalysts that are directly exposed to the electrolyte, presenting opportunities to improve the durability, selectivity, and activity of the active electrocatalyst material. However, the intertwined nature of the physical and chemical phenomena that underlie OEC operation adds complexity to the task of rationally designing OECs, and an improper choice of operating conditions or overlayer characteristics can be highly detrimental to their performance. To help researchers overcome these challenges, this chapter describes (i) the transport and kinetic principles that underlie OEC operation and (ii) experimental tools and methodologies that can be used to evaluate OEC performance. Collectively, the principles and methodologies described here can serve as a framework for developing design rules for optimized OECs that can be applied to a wide range of electrochemical energy applications.