  We report on the optical properties of a sample of 30 BL Lac host galaxies in
the redshift range $0.03<z<0.2$, as derived from HST observations. All galaxies
are fully resolved in the WFPC2 (F702W filter) images, allowing a quantitative
analysis in two dimensions. Most and possibly all these galaxies have
characteristics very similar to those of ``normal'' giant ellipticals. The
luminosity, ellipticity, isophote twisting and amount of disky or boxy
isophotes are consistent with those found in non-active ellipticals and in
radio galaxies. In all cases the BL Lac nucleus is well centered in the main
body of its host galaxy, a result that argues strongly against the microlensing
hypothesis for any significant fraction of the population. A search for faint
sub-structures in the host galaxies has not revealed notable signatures of
tidal distortions or sub-components (faint disks, bars, X features, etc.), and
with only one exception, there are no prominent dusty features in the central
regions. Instead, the BL Lac host galaxies are smooth and unperturbed,
suggesting that strong external gravitational interactions are not important to
ongoing activity. A careful examination of the environment around the nucleus,
however, shows a high incidence of close companion objects, whose nature
remains unclear pending spectroscopic observations.
