The assumption that galaxies trace mass enables us to estimate Ω and to interpret the large-scale structure based on observations of galaxies. However, it would be astonishing if galaxy formation was not affected by environmental effects (e.g. Dressier 1980), resulting in biased galaxy formation. I will summarize the evidence for this suspicion, argue that a segregation of one sort or another is a natural outcome of every cosmogonic scenario (although it is not always of the desired sort), and point at key observational tests before expressing my biased conclusions (see Dekel 1986 for a detailed discussion).