The intergalactic gas in groups and clusters of galaxies bears the indelible stamp of galaxy formation. We present a comparison between observations and simple theoretical models indicating that radiative cooling governs the entropy scale that sets the core radius of the intracluster medium. Entropy measured at the radius 0.1 r_200 scales as T^2/3, in accord with cooling-threshold models for the regulation of core entropy. Cooling of baryons to form galaxies is likely to lead to feedback, and the signature of feedback may appear farther out in the cluster. Entropy measured at the radius r_500 in all but the most massive clusters exceeds the amount that can be generated by hierarchical accretion. However, feedback that smoothes the density distribution of accreting baryons, perhaps via galactic winds, can boost entropy production at the accretion shock by a factor ~ 2-4. An initial comparison of entropy at r_500 to smooth accretion models shows that smooth accretion is a plausible explanation for this excess entropy and suggests that baryon accretion onto groups was smoother than baryon accretion onto clusters.

7 pages, 2 figures, To appear in ApJ Letters,10 Sept 2003