Overlapping-image multimode interference (MMI) couplers, a new class of devices, permit uniform and nonuniform power splitting. A theoretical description directly relates coupler geometry to image intensities, positions, and phases. Among many possibilities of nonuniform power splitting, examples of 1 × 2 couplers with ratios of 15:85 and 28:72 are given. An analysis of uniform power splitters includes the well-known 2 × N and 1 × N MMI couplers. Applications of MMI couplers include mode filters, mode splitters-combiners, and mode converters.