2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/635We review the history and a few features of IC/3K emission. We discuss observing strategies, the physical parameters which can be derived directly or indirectly from successful detections, and the disparities between magnetic field strength estimates found by different methods.4 pages latex with one embedded figure.Published in MPE Report 271, October 1999, page 219. Proceedings of the Ringberg workshop: "Diffuse Thermal and Relativistic Plasma in Galaxy Clusters", editors H. Böhringer, L. Feretti, and P. SchueckerAstrophysicstext