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This paper compares the scalability and energy consumption of switch fabrics for optical and electronic packet switching. In particular, arrayed-waveguide-grating (AWG)-based switches, semiconductor optical amplifier (SOA) gate arrays, electro-optic phased-array switches, and microresonator-based switches are compared with state-of-the-art electronic switch fabrics. The analysis includes the key contributors to energy consumption and the scaling capabilities of each technology. To provide a fair comparison, the analysis takes into account the optical-to-electronic and electronic-to-optical converters and the multiplexers and demultiplexers needed in electronic packet switches. We show that optical switch fabrics generally become more energy efficient as the data rate increases, and AWG and microresonator-based switches consume marginally less energy than electronic switch fabrics at bit rates above about 100 Gb/s. However, optical packet switches do not appear to offer significant throughput improvements or energy savings compared to electronic packet switches. A key impediment to the scaling of optical switch fabrics is the energy consumed in the electronic circuits that drive the individual optical devices in the switch fabric and the energy consumed by the network of control lines that feed these electronic drivers. Ultimately, the energy consumption of optical packet switches is limited by the optical-to-electronic and electronic-to-optical converters required for packet header recognition and header replacement.