Temperature sensing is an important part of implanted biomedical devices. For instance, temperature sensing can be used in various biomedical applications such as health monitoring or smart implants. The accurate measurement of temperature is critical for these applications to detect early signs of diseases or device overheating. It also enables the monitoring of the health status of a patient. Temperature sensing is also required to prevent excessive wireless powering and control thermal requirements inside the body or brain. Also, to minimize the device area, the inclusion of an on-chip temperature sensor is essential. To achieve these goals, temperature sensors can be integrated into each biomedical implant or smart tag. Hence, the development of ultra low-power and high-precision temperature sensors has become an active research area in recent years. A temperature sensor for biomedical applications should exhibit energy and area efficiency and provide maximum temperature sensitivity. Temperature is mainly converted into an analog electrical signal, such as voltage, current, delay, and frequency. Subsequently, this signal can be translated into a digital signal to support thermal management. On-chip temperature sensors are used to measure temperature using a variety of devices, such as resistors, BJTs, MOS gate leakage and MOS transistors. This chapter presents a fully integrated CMOS temperature sensor. It utilizes CTAT/PTAT voltage comparison and a sensitivity improvement circuit. The proposed temperature sensor is designed to operate in the subthreshold region.