This paper presents an overview of the CMOS on-chip rectifiers for RF energy harvesting in the RFID, internet of things, or wearable device applications. The pros and cons of different RF to DC rectifier topologies are investigated, compared, and summarized. All the topologies are designed and optimized in a 65nm CMOS process, with the input power range from -10dBm to 4dBm and the load resistor range from 10kω to 1Mω. The simulated results show that the cross-connected (CC) topology exhibits the highest achievable power conversion efficiency up to 65% (rectifier only), while the CC topology with the threshold voltage cancellation technique and the MOS diode topology achieve 46.7% and 51%, respectively.