Large scale structures have been traced in COSMOS from z = 0.2 to 2.5 in the baryons (from galaxy density distribution) and in the dark matter to z = 1.1 (from weak lensing). These LSS extend over 20 Mpc with total mass up to ∼ 10¹⁵M_⊙. The overall distribution of galaxy overdensities agrees well with those predicted from the Millennium simulation. A trend for an increasingly high overdensity toward low z is clearly apparent in these data. The median galaxy spectral energy distribution (SED) and star formation rate (SFR) is seen to vary systematically with both redshift and environmental density – early type SEDs and lower SFRs in denser regions and at lower redshift. This evolution is probably driven by the exhaustion of the ISM and by galaxy interactions. Strong evolution is seen the frequency of close pairs of galaxies – particularly for lower mass companions at projected separations 10 - 20 kpc.