Ultra-low power/energy efficient wireless sensing microsystems plays an important role in the upcoming internet of things (IoT) era, with emerging application including medical, surveillance, and environmental monitoring and many more. Being one of the most common measurement parameters, high performance CMOS temperature sensors become an important design element of such microsystems. This chapter covers the fundamental knowledge and operating principles of CMOS temperature sensors. The main sources of inaccuracy will be identified, followed by possible circuit/system level solutions to achieve high power/energy efficiencies. Case studies of state-of-the-art CMOS temperature sensors will also be introduced.