  In recent years, a variety of techniques at optical, near-infrared, sub-mm,
and radio wavelengths have opened complementary windows on the high-redshift
Universe. Here we review the current understanding of the general properties of
the z>~2 galaxies detected in the optical (Lyman-break galaxies) and in the
sub-mm (SCUBA sources). We list some of the key questions that need to be
answered in order to understand the nature and evolution of the high-redshift
galaxies. Wherever possible, we present tentative answers given so far to those
questions, in particular on the low-redshift couterparts of the high-redshift
galaxies, on the impact of dust obscuration on the observed quantities, and on
physical characteristics of the high-z systems as inferred from observations.
