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This paper reviews recent progress on CMOS-integrated optical receivers for on-chip interconnects, which have become attractive for achieving communication bandwidth well beyond terabit-per-second with low-power consumption. The design of optical receivers and the performance metrics required from the photodetector (PD) for a low-power receiver is discussed. The progress in waveguide-integrated germanium PDs is reviewed in depth by exploring various optical/electrical designs, and the associated integration approaches for Ge films and metal contacts. The impact of design and integration on PD performance is evaluated by comparing reported results. Finally, the challenges of monolithic integration of PDs within standard CMOS process are discussed.