Although direct sunlight is commonly viewed as incoherent- and therefore ostensibly not suitable for antenna collection-all radiation exhibits spatial coherence when collected on a sufficiently small scale. A first step in evaluating the potential of solar aperture antennas for light harvesting is establishing basic performance bounds based on a generalized analysis of the partial coherence of broadband solar radiation, which comprises a substantial part of this chapter. Indeed, direct sunlight exhibits spatial coherence on a scale that is two orders of magnitude larger than its characteristic wavelengths. This in turn indicates the feasibility of using optical concentrators that can effectively replace antenna and rectifier elements by as much as a factor of 10,000. Our theoretical results quantify the fundamental tradeoff between aperture antenna size and intercepted power, which provides a measure of coherence efficiency. They also illustrate why applying the notion of aperture antennas for collecting radiation from conventional thermal sources is not feasible. Our analytic computations are followed by details of the first direct measurement of the spatial coherence of solar beam radiation, with a novel cyclic-shearing interferometer. These experimental results validate the theoretical predictions, with promising consequences for solar aperture antennas.