A comprehensive study has been conducted into the inception of stall in a multistage high-speed axial flow compressor. The compressor tested was a highly loaded, four-stage axial compressor representing the state of the art in current design technology. Data were taken at a variety of speeds, both with and without inlet distortion. High-frequency response pressure measurements were made on three of the four stages. The data were spatially Fourier analyzed for the existence of pressure waves traveling about the annulus of the compressor prior to stall. In addition, system identification methods were used to estimate the frequency and stability of such waves. In this test, these waves were found at nearly all conditions tested. However, the duration of the waves prior to stall differed markedly as a function of the speed of the compressor. The system identification results showed that the stability of these waves approached zero as the compressor was stalled. LASSICALLY, instabilities in the overall operation of a compressor have been classified as either rotating stall or surge. Both of these conditions result in a substantial decrease of the overall performance of the compressor. In this article, the inception of both rotating stall and surge will be referred to simply as stall, regardless of the final form of the instability. The theory of the formation of compressor stall has continued to evolve since the theory of Emmons et al. 1 was presented. Emmons proposed a mechanism by which a stall cell could propagate around the annulus of the compressor, typically at approximately half the rotational speed of the compressor. More recently, efforts have been made to identify the formation of these stall cells prior to the actual stall event. The linear model of Moore provides a prediction of the properties of small-amplitu de stall cells in a compressor prior to stall.2 Three properties predicted by the model are of particular interest. First, the perturbations of the uniform flowfield are composed of small amplitude sinusoidal waves that travel in the circumferential direction. Second, multiple independent modes of these waves may be found to exist simultaneously. Third, the growth and decay of these modal waves are governed by an exponential function of time. These predictions have been used as a basis to perform investigations into the early identification of compressor stall. Currently, interest exists in the experimental work being conducted in the field of prestall identification. Considerable progress has already been made in the area of low-speed axial compressors. McDougall et al. 3 established the existence of small amplitude waves traveling circumferentially around the annulus of the compressor that develop into stall cells in a continuous manner. In a more detailed study, Gamier et al. 4 confirmed the finding of McDougall in multiple configurations and extended the analysis to provide an experimental estimation of the stability of the compression system. In addition, Day's investigation of prestall phenomena found waves of varying time scales and amplitudes preceding compressor stall.5 Both Paduano et al. 6 and Day7 have used these results to