A wide variety of new experimental diagnostic techniques have been developed to more fully characterize the physical and chemical processes occurring in an IC engine. The advent of lasers has spurred interest in the development and application of optical techniques for nonperturbing, in situ measurements of temperature, species concentration, velocity and turbulence. These supplement and expand the capability of those classical techniques that are reviewed in a companion paper. The new diagnostics are categorized according to the particular part of the engine cycle under investigation: precombustion fluid motion, fuel preparation, combustion, and emission formation. Current applications of each technique to engine experiments are surveyed and put into the perspective of resolving critical issues facing the engine design community.