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The startling success of GaN-based light emitting diodes despite the high density of dislocations found in typical heteroepitaxial material has been attributed to localization of carriers at non-uniformities in the quantum wells (QWs) which form the active region of such devices. Here, we review the different possible structures within the QWs which could act as localization sites, at length scales ranging from the atomic to the tens of nanometre range. In some QWs several localization mechanisms could be operational, but the challenge remains to optimize the QWs' structure to achieve improved quantum efficiencies, particularly at high excitation powers.