  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.
