We compare the energy consumption of digital optical and digital electronic signal processing circuits, including the contributions to energy consumption of the optical to electrical (O/E) converters and electrical to optical (E/O) converters, and the demultiplexers (DEMUXs) and multiplexers (MUXs) required for electronic circuits to process high-speed optical signals. This paper focuses on three key practical considerations, namely, energy consumption, energy density, and the complexity of processing. We show that optical signal processing is potentially competitive with electronics in very high-speed circuits that provide only limited processing, i.e., when only a small number of processing operations are performed on each bit of data. However, in applications that require anything more than limited processing, electronics provides better energy efficiency and occupies a smaller footprint. In these applications, electronics is likely to remain the technology of choice. More attention needs to be paid to energy consumption issues in the research and development of new digital optical technologies.