Using Hubble Space Telescope and WFPC2 we obtained short exposure images for a hundred of BL Lac objects. The targets were centered mostly in the PC using the F702W filter. Sources from various samples, including objects of very different redshift (0.05 to 1.2), have been observed. The high resolution and homogeneity of the images allow us to address the properties of the immediate environments of BL Lacs with unprecedented capability. We report here the results on the host galaxies and compare their properties with previous ground based studies. A number of peculiar features detected, that need follow up investigation, are also briefly outlined.