Dressler et al. 1980; Lewis et al. 2002; G[' o]{}mez et al. 2003). One can also use kernel density estimators ([*e.g.*]{}, Eisenstein 2003) and mark correlation functions.

There were many great talks and posters on the topic of galaxy evolution in clusters at this conference. For example, see the contributions by Bower, Davis, Dressler, Franx, Goto, Koo, Kauffmann, Martini, N. & C. Miller, Postman, Poggianti, Tran and Treu. Also, I refer the reader to the work of Hogg et al. (2003) who is also using the SDSS data to study the effects of environment on the colors, surface brightnesses, morphologies and luminosities of galaxies.

### Critical Density

Is the star–formation rate (SFR) of a galaxy affected by its environment? The answer appears to be yes, and was discussed by several authors at this conference (see the contributions by Miller, Couch and Bower). In particular, G[' o]{}mez et al. (2003) find that the fraction of strongly star–forming galaxies in the SDSS decreases rapidly beyond a critical density of $\simeq 1\,h_{75}^2\,{\rm
Mpc^{-2}}$ (see also Lewis et al. 2002). This result is demonstrated in Figure \[critical\] and appears to be the same for all morphological types (see G[' o]{}mez et al. 2003 and 