Artificial Bandwidth Extension (ABWE) algorithms try to reconstruct frequency components of speech signals which have been lost by coding/transmission. From a system theoretical point of view, this seems to be questionable. However, by exploiting redundancy of speech production as well as psychoacoustic properties of auditory perception, missing parts can subjectively be recovered or at least mimicked to a certain extent. The motivation for ABWE is to bridge the quality gap between the traditional narrowband telephony (200–3400 Hz) and the recent wideband services (HD voice, 50–7000 Hz). An HD terminal which is connected to a traditional phone, may activate ABWE for improving the audio quality and possibly the intelligibility of the received signal. In this contribution, state-of-the-art ABWE algorithms and coding standards including ABWE techniques are reviewed. ABWE algorithms are based on a source-filter model. The three main sub-tasks are to estimate the excitation signal, the spectral envelope and the time domain envelope, given either just the narrowband signal or in addition a few bits of side information. Furthermore, insights with respect to the relative importance of these three components are presented.