An optical wireless communication system comprising: a first device comprising a transceiver apparatus; and a plurality of further devices (14a 14b, 14c) each comprising respective further transceiver apparatus, wherein the first device is configured to communicate via at least one optical channel with the plurality of further devices (14a 14b, 14c), and the transceiver apparatus of the first device comprising: a receiver for receiving light representing optical wireless communication signals transmitted by the further devices (14a 14b, 14c), the receiver comprising at least one photodetector; receiver-side processing circuitry (26, 28) for processing optical wireless communication signals received by the receiver to extract data represented by the received optical wireless communication signals; a transmitter for transmitting further light representing optical wireless communication signals; transmitter-side processing circuitry for producing optical wireless communication signals for transmission by the transmitter; a multiplexer arrangement that is arranged to receive optical wireless communication signals from a plurality of signal paths and to pass the signals to the transmitter for transmission; and a controller for controlling operation of the transceiver apparatus of the first device, wherein a first of the signal paths to the multiplexer arrangement is from the transmitter-side processing circuitry; a second of the signal paths to the multiplexer arrangement is from the receiver-side; the controller controls operation of the transceiver apparatus so as to pass via the multiplexer arrangement to the transmitter for re-transmission at least part of an optical wireless communication signal received from one of the further devices by the receiver thereby to indicate to the other further devices of the plurality of further devices (14a 14b, 14c) that an optical channel of the receiver is busy; and the controller is operable to control whether the optical wireless communication signals received from one of the further devices (14a 14b, 14c) are sent to both or either or neither of the multiplexer arrangement and a component of a receiver-side processing chain. a first device comprising a transceiver apparatus; and a plurality of further devices (14a 14b, 14c) each comprising respective further transceiver apparatus, wherein the first device is configured to communicate via at least one optical channel with the plurality of further devices (14a 14b, 14c), and the transceiver apparatus of the first device comprising: a receiver for receiving light representing optical wireless communication signals transmitted by the further devices (14a 14b, 14c), the receiver comprising at least one photodetector; receiver-side processing circuitry (26, 28) for processing optical wireless communication signals received by the receiver to extract data represented by the received optical wireless communication signals; a transmitter for transmitting further light representing optical wireless communication signals; transmitter-side processing circuitry for producing optical wireless communication signals for transmission by the transmitter; a multiplexer arrangement that is arranged to receive optical wireless communication signals from a plurality of signal paths and to pass the signals to the transmitter for transmission; and a controller for controlling operation of the transceiver apparatus of the first device, wherein a receiver for receiving light representing optical wireless communication signals transmitted by the further devices (14a 14b, 14c), the receiver comprising at least one photodetector; receiver-side processing circuitry (26, 28) for processing optical wireless communication signals received by the receiver to extract data represented by the received optical wireless communication signals; a transmitter for transmitting further light representing optical wireless communication signals; transmitter-side processing circuitry for producing optical wireless communication signals for transmission by the transmitter; a multiplexer arrangement that is arranged to receive optical wireless communication signals from a plurality of signal paths and to pass the signals to the transmitter for transmission; and a controller for controlling operation of the transceiver apparatus of the first device, wherein a first of the signal paths to the multiplexer arrangement is from the transmitter-side processing circuitry; a second of the signal paths to the multiplexer arrangement is from the receiver-side; the controller controls operation of the transceiver apparatus so as to pass via the multiplexer arrangement to the transmitter for re-transmission at least part of an optical wireless communication signal received from one of the further devices by the receiver thereby to indicate to the other further devices of the plurality of further devices (14a 14b, 14c) that an optical channel of the receiver is busy; and the controller is operable to control whether the optical wireless communication signals received from one of the further devices (14a 14b, 14c) are sent to both or either or neither of the multiplexer arrangement and a component of a receiver-side processing chain.