  We discuss alternative interpretations of the differences in the Spectral
Energy Distributions (SEDs) of BL Lacs found in complete Radio or X--ray
surveys. In order to explain the different properties of radio and X-ray
selected within the assumption of a single population we propose a new approach
The model is based on the idea that the BL Lacs constitute a ``parameterized
family'' and that the physical parameter which governs the shape of the SEDs,
is (or is associated with) the bolometric luminosity. Assuming an empirical
relation between spectral shape and luminosity the observational properties of
the two surveys, including the redshift distributions, can be reasonably well
reproduced.
