A stepped light guide illumination system employing a planar sheet of an optically transmissive material having a stepped light guiding structure including one or more parallel arrays of channels. Light is input into the stepped light guiding structure using a number of light sources, such as LEDs, located within the channels and coupled to the stepped light guiding structure at multiple locations. Light is extracted from the stepped light guiding structure by two-dimensional patterns of light extraction features provided in surface areas of the stepped light guiding structure located between the parallel channels.