Wavelets have been established as an essential tool for the analysis of non-stationary signals. This paper presents a novel approach to the analysis of non-intrusive pressure traces using wavelets. It starts with a background to wavelets, how they might be useful for the analysis of signals from internal combustion engines and some results obtained from the analysis of the non-intrusive in-cylinder pressure data.The signals obtained from the non-intrusive in-cylinder pressure sensors contain random and statistical noise. They are also contaminated with reflections and resonances from the structures within the engine. Results presented at SAE99 [ref 1] show that the signals contain useful information for the determination of timing and pressures. The purpose of this paper is to present a novel approach to the processing and analysis of these signals.