Issues regarding combining of laser diodes for application to free-space optical communication systems are examined. It is shown that to deliver a greater number of photons to a distant collector the combining technique must use wavelength and/or polarization. Optical elements for combining include multilayer dielectric interference filters (bandpass, dichroic, and polarizing), diffraction gratings, and thin film evaporative metal polarizers. Design considerations for a combiner include high throughput efficiency, diffraction-limited angular divergence, and reasonable volume constraints. It is found that combiners require a generalized concept of Strehl Ratio which includes relative source misalignment, and that diffraction grating combiners may have a limited number of laser sources which can meet the spectral requirements. Next, given an optical system for laser diode combining, methods to incorporate the combiner into the communication system are compared. The communication system alternatives include power combining, multiplexing parallel independent channels, or a generalized matrix modulation scheme. Power combining may be limited by background radiation, and matrix modulation may be limited by the source peak-to-average power ratio. Parallel channels does not have these limitations but requires the most hardware to implement. It is concluded that power combining is the best trade-off of performance and complexity for all systems except those severely limited by background radiation or component bandwidth.