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Ever since the discovery of ultra-broadband near-infrared (NIR) photoluminescence (PL) from Bi-doped silicate glass, this class of materials and corresponding devices have experienced rapid progress. This is mainly driven by the suggested use in broadband optical amplifiers and novel lasers for future telecommunication networks. Currently, it appears that the optical bandwidth which is provided by Bi-doped glasses and crystals cannot be achieved by any rare-earth (RE) based amplifier, or by the combination of multiple RE-doped devices. However, the nature of the optically active NIR emission centers remains highly debated. The present paper critically reviews the various arguments and models which have been proposed in this context over the last decade. From the overall conclusions, the major open questions are identified.