In most situations, the fastest and most cost-effective approach to providing high-tier electricity access is by extending or enhancing the existing grid. For this reason, grid extension is the default approach to increasing electricity access. However, in situations where an off-grid community is remote, far from the existing grid, sparsely populated, and whose electricity consumption is anticipated to be low, off-grid solutions can be preferable. This chapter describes grid extension as a method for providing access to electricity. The design and operation of medium-voltage distribution lines are discussed. An economic model considering substations, medium- and low-voltage lines, transformers, and user metering and wiring is developed. The model allows the grid extension approach to be compared to off-grid approaches. The role of Rural Electrification Authorities in grid extension is detailed.