  BL Lac objects are active nuclei, hosted in massive elliptical galaxies, the
emission of which is dominated by a relativistic jet closely aligned with the
line of sight. This implies the existence of a parent population of sources
with a misaligned jet, that have been identified with low-power radiogalaxies.
The spectrum of BL Lacs, dominated by non-thermal emission over the whole
electromagnetic range, together with bright compact radio cores, high
luminosities, rapid and large amplitude flux variability at all frequencies and
strong polarization make these sources an optimal laboratory for high energy
astrophysics. A most distinctive characteristic of the class is the weakness or
absence of spectral lines, that historically hindered the identification of
their nature and ever thereafter proved to be a hurdle in the determination of
their distance. In this paper we review the main observational facts that
contribute to the present basic interpretation of this class of active
galaxies. We overview the history of the BL Lac objects research field and
their population as it emerged from multi-wavelength surveys. The properties of
the flux variability and polarization, compared with those at radio, X-ray and
gamma-ray frequencies, are summarized together with the present knowledge of
the host galaxies, their environments, and central black hole masses. We focus
this review on the optical observations, that played a crucial role in the
early phase of BL Lacs studies, and, in spite of extensive radio, X-ray, and
recently gamma-ray observations, could represent the future major contribution
to the unveiling of the origin of these sources. In particular they could
provide a firm conclusion on the long debated issue of the cosmic evolution of
this class of active galactic nuclei and on the connection between formation of
supermassive black holes and relativistic jets.
