We report the result of the HST snapshot survey of BL Lac host galaxies. Almost 100 sources with redshift ranging from 0.05 to 1.2 were observed. Virtually all source at z<0.5 are resolved and the host can be studied well enough that the morphological type is reliably determined. The BL Lac hosts are, without (or very few) exception, bright, giant elliptical with luminosity within a narrow range of values (-22.3 < M(R) < -25). No significant cosmological evolution is observed up to z=3D0.5. Results are discussed in view of unified models.