Because cylinder pressure is the means by which the reciprocating internal combustion engine transforms the chemical energy released in combustion into useful mechanical work, it is not surprising that the measurement of cylinder pressure has been an important aspect of research on that engine since its earliest days. Acquiring data of sufficient accuracy for many quantitative uses is not a simple task. Some of the precautions that have influenced the development of pressure instrumentation and must still be exercised today are briefly considered. Then some of the techniques used for applying those measurements in engine research are reviewed. Such uses include the evaluation of the distribution of cycle work, the estimation of net heat release during combustion, the assessment of cycle-to-cycle variability, and the diagnosis of abnormal combustion. Although the examples presented are drawn from research on spark-ignition engines, most of the information in this paper also applies to diesel engines.