Currently there are three quite different views about galaxy evolution, each one improving the previous state of knowledge:

1. (1) The older one (“ELS”) in which galaxies form by collapse early, quickly, and synchronously (during the “galaxy formation epoch”), ending the dynamically active period; subsequent galaxy evolution is merely a matter of stellar formation processes in a rigid potential.

2. (2) An alternative one (“SZ”) in which disks are viewed as forming inside out over an extended period of time. Galaxy evolution occurs without important internal dynamical instabilities.

3. (3) The slowly emerging picture, after 40 years of N-body simulations and the obvious evidences from recent high-z observations: galaxies evolve both dynamically and chemically over most of the Hubble time in a widely asynchronous way at different speeds, depending on the environment. The Hubble sequence, from late to early types, appears to represent a broad description of the general aging process.

Thus galaxies appear now as evolving structures over typical time-scales of order of 1 Gyr. A fundamental aspect of the micro-physics in galaxies is star formation and gas processes in which the H2 molecule must play a key role: indeed interstellar gas must first form H2 before being able to form stars, so star forming regions do trace molecules, although CO might not have been detected.