  Snapshot images of about 100 BL Lac objects were obtained with WFPC2 on HST.
Sources from various samples, in the redshift range 0.05 to 1.2, were observed
and 61 resolved (51 with known z).
  The high resolution and homogeneity of the images allow us to address the
properties of the immediate environments of BL Lacs with unprecedented
capability. Host galaxies of BL Lacs are luminous ellipticals (on average 1 mag
brighter than L*) with no or little disturbed morphology.
  The nucleus, that is always well centered onto the galaxy, contributes in the
optical (R band) to about half of the total luminosity of the object (range of
the contribution from 0.1 to 10). The undisturbed morphology suggests that the
nuclear activity has marginal effect on the overall properties of the hosts.
Nonetheless several examples of close companions have been detected.
  The luminosity distribution of host galaxies is compared with that of a large
sample of FR-I radio galaxies.
