Intergalactic matter and the evolution of galaxies

The current status of observations of gas within clusters and superclusters of galaxies, and perhaps outside of superclusters, is reviewed, with emphasis on the interpretation of X-ray emission by hot gas and the physical effects of such gas upon galaxies. It is concluded that while interfalactic gas probably makes a contribution to the mean desnity of the universe which is to for closure, it can nevertheless profoundly affect the evolution universe which is too small fro closure, it can nevertheless profoundly affect the evoluation of galaxies eitherby sweeping out their interstellar gas or by accreting on the to them.