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This paper reviews recent progress on high-density and large-scale arrayed-waveguide-grating (AWG) multi/demultiplexers, which have been developed for wavelength division multiplexing (WDM)-based photonic networks. The AWG has been the key to the construction of flexible and large-capacity WDM networks. This is because, compared with conventional filters consisting of thin-film interference filters and microoptics, the AWG offers the advantages of low loss, high port counts, and mass productivity. To improve such characteristics further, low-loss, higher index-contrast (super-high Δ) planar lightwave circuits (PLCs) with a bending radius of 2 mm have recently been developed. It has been shown that these PLCs are effective for use in constructing a compact AWG module with 1/5 the volume of a conventional module and large-scale AWGs with 256 and 400 channels. Three techniques for low-loss fiber connection with spot-size converters have also been developed for the super-high Δ PLCs, and it has been confirmed that these techniques can be applied to the fabrication of AWG modules. Furthermore, two-stage tandem AWG-type multi/demultiplexers with more than 1000 channels have been demonstrated. This paper describes the progress that has been made on these high-density and large-scale AWGs, which are expected to contribute greatly to the construction of future photonic networks including optical add/drop multiplexing systems and optical crossconnect systems.