COSMOSOMAS is a circular scanning astronomical microwave experiment to investigate the Cosmic Microwave Background anisotropy and diffuse emission from the Galaxy on angular scales from 1 to 5 degrees. It was designed and built by the Instituto de Astrofísica de Canarias (IAC) in Tenerife, Spain, in 1998. Its name comes from "COSMOlogical Structures On Medium Angular Scales" referring to CMB fluctuations. This experiment grew out experience of the previous Tenerife Experiment with the need to go to smaller angular scales with greater sensitivity.