  We present the results of a VLT and HST imaging survey aimed at the
identification of 4.5<z<6 galaxies. In the VLT data, a set of broad and
intermediate band filters has been used to select 13 high--$z$ candidates in a
Z(AB)<25 catalog, over an area of about 30 arcmin2. Discrimination against
lower redshift interlopers (mainly early--type galaxies at high redshift and
cool Galactic stars) has been done combining morphological and spectral
classification. This sample has been combined with a deeper I<27.2 sample
obtained from the Hubble Deep Field campaigns. The VLT final sample consists of
13 high z candidates, 4 of which are identified with high confidence as z>4.5
galaxies. The resulting integral surface density of the Z<25 candidates at
z>4.5z is in the range 0.13-0.44/arcmin^2 and that in the highest redshift bin
5<z<6 is between 0.07-0.13/arcmin^2. In the two HDFs, we identify at I<27.2 25
galaxies in the range 4.5<z<5 and 16 at 5<z<6$, corresponding to surface
densities of 3.1 arcmin^-2 and 2 arcmin^-2, respectively. We show that the
observed Z<25 UV luminosity density appear to drop by about one order of
magnitude from z~3 to z~6. However, if we apply a threshold to obtain an
absolute--magnitude limited sample, the UV luminosity density results to be
roughly constant up to z~6. We finally show that recent semi-analytic
hierarchical models for galaxy formation, while predicting a nearly constant
total UV luminosity density up to z~6, under-predict the observed UV luminosity
density at Z<25 and over-predicts the I<27.2 one. This behaviour can be
understood in term of a poor match to the slope of the UV luminosity function.
