Negative Bias Temperature Instability (NBTI) is one of the most critical device reliability issues facing scaled CMOS technology. In order to better understand the characteristics of this mechanism, accurate and efficient means of measuring its effects must be explored. In this work, we describe an on-chip NBTI degradation sensor using two delay-locked loops (DLL). The increase in PMOS transistor threshold due to NBTI stress is translated into the control voltage of a DLL for high sensing gain. Measurements from a 0.13m test chip show a maximum gain of 16X in the operating range of interest, with microsecond order measurement times for minimal unwanted recovery. The proposed NBTI sensor also supports various DC and AC stress modes.