  Optical light curves of bright BL Lac objects, in particular those having the
SED synchrotron peak at frequencies lower than 10E14 Hz, are characterized by
long-term trends with time scales of a few decades. These variations are
probabily not related to fast perturbations moving down the jet, which have a
much faster evolution, but more likely to slow changes of other physical or
geometrical parameters, for example the jet direction. In these cases we can
expect changes of the inner jet structure detectable with VLBI imaging. Recent
literature results are reviewed and discussed in this context.
