Visible Light Communication (VLC) systems often provide wavelength diversity by using white LED sources as transmitters or spatial diversity by simultaneous transmission from multiple LEDs. This paper focuses on optimum diversity combining techniques for VLC systems employing intensity modulation/direct detection (IM/DD) with binary signaling. VLC systems can combine the signals from separate channels with the use of optical filters and special optics either in the optical domain using one photodetector only, or in the electrical domain by allocating a dedicated photodetector per channel. We show that, in the case of optical combining, maximization of the SNR is achieved by a fundamentally different solution compared to the well-known maximal-ratio combining (MRC) in wireless radio. In the case of electrical combining, the classic MRC solution remains optimum. Finally, when photon noise is dominant, it is proven that optimum combining in the electrical domain outperforms combining in the optical domain for the same light input.