  A review of gravitational and hydrodynamical processes during formation of
clusters and evolution of galaxies is given. Early, at the advent of N-body
computer simulations, the importance of tidal fields in galaxy encounters has
been recognized. Orbits are crowded due to tides along spiral arms, where the
star formation is enhanced. Low relative velocity encounters lead to galaxy
mergers. The central dominating galaxies in future clusters form before the
clusters in a merging process in galaxy groups. Galaxy clusters are composed in
a hierarchical scenario due to relaxation processes between galaxies and galaxy
groups. As soon as the overall cluster gravitational potential is built, high
speed galaxy versus galaxy encounters start to play a role. These harassment
events gradually thicken and shorten spiral galaxy disks leading to the
formation of S0 galaxies and ellipticals. Another aspect of the high speed
motion in the hot and diluted Intracluster Medium (ICM) is the ram pressure
exerted on the Interstellar Matter (ISM) leading to stripping of the ISM from
parent spirals. The combinations of tides and ram pressure stripping
efficiently removes the gas from spirals, quenching the star formation in
galactic disks, while triggering it in the tidal arms and at the leading edge
of gaseous disk. Gas stripping from disks transports the metals to the ICM. In
some cases, the gas extracted from the galactic disks becomes self-gravitating
forming tidal dwarf galaxies.
