Bluetooth is a short-range wireless technology used for the exchange of data between devices, fixed or mobile, over short distances (a few meters up to about 240 m). It operates in the 2.4-GHz unlicensed band and is primarily used as an alternative to wired connections to link, for example, smartphones and music players to headphones and speakers. In this chapter, emphasis is placed on the latest two versions of Bluetooth, namely versions 5 and 6 which fall under the nomenclature of Bluetooth Low Energy or BLE. A key feature of BLE is that it is highly efficient in its use of power. This allows a new class of devices that are powered by small coin-sized batteries and that can operate for days, weeks, or even longer without a battery change. In addition, BLE supports not only point-to-point topology, but also mesh and broadcasting topologies. In this chapter, we examine, for Bluetooth 5/6, the protocol architecture, the RF spectrum supported, the physical and link layers (LLs), and the key technologies employed therein, and finally a comparison between Bluetooth 5/6 and earlier versions known collectively as Bluetooth Classic is presented.