SiGe HBTs (heterojunction bipolar transistors) are a type of Si-based BJTs (bipolar junction transistors) that employ a SiGe alloy layer as a part of the device, typically as the base layer. Since its first demonstration in 1987, SiGe HBTs have become a mainstream Si bipolar transistor and are now widely employed for various high-frequency applications. In this chapter, a brief overview of SiGe HBTs is provided. Firstly, the DC characteristics are described in terms of the collector current and current gain, with focus on the comparison with Si BJTs and the origin of the advantages. A review on the definition of various breakdown voltages will be also presented along with device parameters that affect the breakdown voltages. Secondly, the RF characteristics of SiGe HBTs will be discussed in terms of fT (cutoff frequency) and fmax (maximum oscillation frequency). Additional discussion will be made on high-frequency noise and 1/f noise of SiGe HBTs. Lastly, a couple of exemplary SiGe HBTs and circuits will be introduced to provide the practical aspect of modern high-performance SiGe HBTs.