Analyses of strong lensing galaxies have provided constraints on
quantities as important and varied as the Hubble constant and substructure
in the dark matter halos of galaxies. However, the environments in which
most lens galaxies lie are not well-known and may contribute significantly
to the lensing potential. Not including the environment in lens models can
introduce uncertainties and biases of 5-20%, preventing lensing
constraints from competing with those derived from CMB or distance ladder
measurements. I will describe results from our theoretical modeling of
environment effects and from our five-year observational survey of the
fields of 70 lenses. We find that the majority of lenses lie in overdense
environments -- poor groups or rich clusters of galaxies -- that perturb
lens models. We are using these data not only to identify "golden lenses"
whose properties provide useful cosmological constraints, but also to
measure the evolution of groups and clusters over the large redshift range
probed by our sample. I will discuss some of our recent results measuring
the baryon content of nearby groups and clusters, which suggest that our
lens survey will ultimately allow us to explore baryon evolution in such
common environments.