This paper provides an overview of electron microscopy observations of epitaxial HgCdTe alloys. Growth of high quality Hg1-xCdxTe epilayers for infrared (IR) detector applications requires a detailed knowledge and control of the experimental factors causing defective material. The type of substrate. the substrate orientation, the Substrate temperature during growth, and the Hg/Te flux ratio are factors that have a significant effect on the film morphology. Extensive characterization studies using electron microscopy methods have provided invaluable information about the connection between defect formation and the influence of specific growth parameters. The types of defects observed by electron microscopy include dislocations, twins and stacking faults, surface hillocks and crater defects. and precipitates. as well as spurious effects induced by sample preparation methods. By combining electron microscopy observations with other characterization methods such as in situ ellipsometry. Fourier Transform IR spectroscopy, and hole measurements, it should be possible to improve the quality of H,,CdTe epilayers still further to meet the demanding requirements of future generation large format focal-plane arrays. (C) 2004 Elsevier B.V. All rights reserved.