The indicator was an important factor in the early development of the internal-combustion engine when engine speeds were low, but on high-speed engines such indicators were unable to reliably reproduce records because of the inertia effects of the moving part of the pressure element. The first need is for a purely qualitative indicator of the so-called optical type, to secure a complete and instantaneous mental picture of the pressure events of the cycle; the second need is for a purely quantitative instrument for obtaining an exact record of pressures. The common requirements for both are that the indicator timing shall correctly follow the positions of the crank and that the pressure recorded shall agree with the pressures developed within the combustion space. Following a discussion of these requirements, the author then describes the demonstration made of two high-speed indicators, inclusive of various illustrations that show the apparatus, and comments upon its performance. In conclusion, a description is given of a high-speed diaphragm indicator of the balanced-pressure type, developed at the Bureau of Standards.