The negative-bias temperature instability (NBTI) is a reliability problem that, in the last ten years, has risen from relative obscurity to become the most important reliability problem in advanced pMOSFET devices. Even though a significant effort has been spent trying to eliminate NBTI signatures (negative threshold voltage shift and transconductance degradation after inversion gate stress at elevated temperatures), the issue still persists. NBTI s elusiveness is due to the fact that NBTI-induced degradation relaxes very quickly after the conclusion of stress. This makes NBTI characterizations quite tedious and clouds the fundamental understanding of the degradation/relaxation mechanism. In high-k gate stacks, the situation is complicated further by inherent fast charge trapping issues and the emergence of an additional positive bias temperature instability (PBTI) component. While this depiction of NBTI may seem hopeless, there have been renewed efforts to uncover the fundamental physical mechanisms which govern this process. This tutorial summarizes the noteworthy NBTI experimental observations/techniques and discusses how these observations might be leading towards a fundamental understanding of NBTI. A general aim is to examine how these observations validate/invalidate the current understanding of NBTI.