PHOENIX is a computer program for segmenting images into homogeneous closed regions. It uses histogram analysis, thresholding, and connected-components analysis to produce a partial segmentation, then resegments each region until various stopping criteria are satisfied. Its major contributions over other recursive segmenters are a sophisticated control interface, optional use of more than one histogram-dependent intensity threshold during tentative segmentation of each region, and spatial analysis of resulting subregions as a form of “look-ahead’’ for choosing between promising spectral features at each step. PHOENIX was contributed to the DARPA Image Understanding Testbed at SRI by Carnegie-Mellon University. This report summarizes applications for which PHOENIX is suited, the history and nature of the algorithm, details of the Testbed implementation, the manner in which PHOENIX is invoked and controlled, the type of results that can be expected, and suggestions for further development. Baseline parameter sets are given for producing reasonable segmentations of typical imagery.