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Spatial Modulation (SM) is a newly introduced wireless transmission scheme, and Space Shift Keying (SSK) is its simplified version. Their capacity performance has not been well examined to date. Starting from basic definitions of information theory, through theoretical analyses and numerical calculations, this paper evaluates the capacity of SM systems in various channels. Our conclusion is that under Rayleigh fading channels, the capacity of the SSK system is limited by the Signal-to-Noise Ratio (SNR) or the number of transmit antennas, whichever one is smaller. From the capacity point of view, the SM system is better than the Single-Input and Single-Output (SISO) system, but worse than the Multiple-Input and Single-Output (MISO) system. The correlation between transmit antennas will degrade the performance of the SSK system. Line-of-Sight (LOS) may cause performance degradation for SSK, but capacity increases for SM.