  A clear insight of the galaxies hosting active galactic nuclei is of
fundamental importance for understanding the processes of galaxies and nuclei
formation and their cosmic evolution. A good characterization of the host
galaxies properties requires images of excellent quality in order to
disentangle the light of the galaxy from that of the bright nucleus. To this
aim HST has provided a major improvement of data on QSOs (Disney et al. 1995;
Bahcall et al. 1996, 1997; Boyce et al. 1998; McLure et al. 1999; Hamilton et
al. 2000; Kukula et al. 2001) and BL Lacs (Scarpa et al. 2000, Urry et al.
2000). We present a comparative study of low redshift QSO and BL Lac host
galaxy luminosity function (HGLF). To this aim we have considered samples of BL
Lacs (Urry et al. 2000) and QSOs (Bahcall et al. 1997; Boyce et al. 1998;
McLure et al. 1999) that have been well resolved by images obtained with WFPC2
on board of HST.
