There is little doubt that the last few years have been very exciting times in galaxy formation and evolution studies. The remarkable progress in our understanding of faint galaxy data made possible by the combination of Hubble Space Telescope (HST) deep imaging and ground-based spectroscopy has permitted to shed new light on the evolution of the stellar birthrate in the universe, to identify the epoch 1 z 2 where most of the optical extragalactic background light was produced, and to set important contraints on galaxy evolution scenarios. The explosion in the quantity of information available on the high-redshift universe at optical wavelengths has been complemented by the detection of the far-IR/sub-mm background by DIRBE and FIRAS onboard the COBE satellite, and by theoretical progress made in understanding how cosmic structure forms from initial density fluctuations . The IR data have revealed the ``optically-hidden'' side of galaxy formation, and shown that a significant fraction of the energy released by stellar nucleosynthesis is re-emitted as thermal radiation by dust. The underlying goal of all these efforts is to understand the growth of cosmic structures, the internal properties of galaxies and their evolution, the mechanisms that shaped Hubble's morphological sequence, and ultimately to map the transition from the cosmic `dark age' to a ionized universe populated with luminous sources. While one of the important questions recently emerged is the nature (starbursts or active galactic nuclei?) and redshift distribution of the ultraluminous sub-mm sources discovered by SCUBA, of perhaps equal interest is the possible existence of a large population of faint galaxies still undetected at high redshifts, as the color-selected ground-based and Hubble Deep Field (HDF) samples include only the brightest and bluest star-forming objects. In any hierarchical clustering (`bottom-up') scenario (the cold dark matter model being the best studied example), subgalactic structures are the first non-linearities to form. High-z dwarf galaxies and/or mini-quasars (i.e. an early generation of stars and accreting black holes in dark matter halos with circular velocities vc ~ 50 km s-1) may then be one of the main source of UV photons and heavy elements at early epochs.
In this talk I will focus on some of the open issues and controversies surrounding our present understanding of the history of the conversion of cold gas into stars within galaxies, and of the evolution of luminous sources in the universe. An Einstein-deSitter (EdS) universe (M = 1, = 0) with h = H0 / 100 km s-1 Mpc-1 = 0.5 will be adopted in the following.