In advanced CMOS technologies, the negative bias temperature instability (NBTI) phenomenon in pMOSFETs is a major reliability concern as well as a limiting factor in future device scaling. Recently, much effort has been expended to further the basic understanding of this mechanism. This tutorial gives an overview of the physics of NBTI. Discussions include such topics as the impact of NBTI on the observed changes in the device characteristics as well as the impact of gate oxide processes on the physics of NBTI. Current experimental results, exploring various NBTI effects such as frequency dependence and relaxation, are also discussed. Since some of the recent work on the various NBTI effects seems contradictory, focus is placed on highlighting our current understanding, our open questions and our future challenges.