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Medium-voltage (MV) sensors for the Smart Grid are an evolving tool for the electric utility distribution engineer. Historically, to remotely monitor the power system flow on a distribution system, the most common option available was traditional current transformers (CTs) and potential transformers (PTs) combined with transducers. The most common means of communication was a leased telephone circuit. Around 1990, as the visions of fully automated distribution systems began to surface, a need was recognized for a device that could detect the magnitudes of the distribution system's primary voltages and line currents - devices that would be less expensive and less cumbersome to install and operate than the traditional CT and PT. Thus was born the MV sensor. As with many other emerging technologies, the evolution of these MV sensors - from the concept of a cycloalaphatic or epoxy post-type insulator with embedded discrete components to the more recent concept of a polymer with optics - has been filled with trials and tribulations. This paper covers some of the basic design concepts, technical capabilities, applications, reports of independent lab tests, and “lessons learned” with these sensor types, which are destined to become part of the initial building blocks for the Smart Grid.