We make observational studies of galaxy properties in various environments, particularly in clusters of galaxies and in galaxy-scale halos. The physical parameters of galaxies studied include luminosity, morphology, star formation activity parameters (color, equivalent width of the H-alpha line, color gradient), and internal structure parameters (concentration, stellar velocity dispersion, size). The dependence of these parameters on small and large-scale environments is studied, and the roles of the gravitational and hydrodynamical interactions in determining galaxy properties are inferred. We find that hydrodynamic interactions between galaxies are ubiquitously important in the fate of galaxies.