We investigate the properties of the host galaxies of X–ray selected ( high frequency peaked ) BL Lac objects using a large and homogeneous data set of high spatial resolution R –band observations of 52 BL Lacs in the EMSS and Slew samples .
The redshift distribution of the BL Lacs ranges from z = 0.04 to z > 0.7 , with average and median redshifts z = 0.26 and z = 0.24 , respectively .
Eight objects are at unknown redshift .

We are able to resolve 45 objects out of the 52 BL Lacs .
For all the well resolved sources , we find the host to be a luminous elliptical galaxy .
In a few cases a disk is not ruled out but an elliptical model is still preferred .

The average absolute magnitude of the host galaxies is < M _ { R } ( host ) > = –23.9 \pm 0.6 , while the average scale length of the host is < R ( e ) > = 9 \pm 5 kpc .
There is no difference in the host properties between the EMSS and Slew samples .
We find a good agreement between the results derived by the surveys of Wurtz et al .
( ground-based data ) and Urry et al .
( HST data ) , and by our new deeper imaging .
The average luminosity of the BL Lac hosts is between those of F-R I and F-R II radio galaxies in Govoni et al. , supporting the idea that both radio galaxy types could contribute to the parent population .
The BL Lac hosts follow the F-P relation for giant ellipticals and exhibit a modest luminosity evolution with redshift .
Finally , we find a slight correlation between the nuclear and host luminosity and a bimodal distribution in the nuclear/host luminosity ratio .