This chapter presents the noncontact and noninvasive approach for monitoring human vital signs using Doppler radar sensor. The theory of noncontact vital sign detection is based on Doppler phase modulation. The system-level hardware architectures of the Doppler radar sensor have been illustrated. Research groups all over the world have been working for decades to improve the performance of Doppler radar noncontact vital sign detection. While some groups improved the performance from the hardware side, e.g., using quadrature receiver to avoid the null point problem, some groups took effort from the software side. For example, new demodulation techniques such as complex demodulation have been invented. System-on-chip (SoC) integration is also a major interest of the researchers working on Doppler radar sensors. With all the circuits integrated into one chip, the radar can be much smaller so as to be more portable or more easily integrated with other communication devices, e.g., cell phones. Several examples of radar sensor SoC have been introduced in this chapter. The Doppler radar sensor sees a bright future in medical applications. It is expected to bring enhanced healthcare in the near future. This chapter introduces the potential applications such as infants monitoring, sleep apnea detection, pulse wave velocity measurement, respiration measurement in motion-adaptive cancer radiotherapy, and so forth.