Very Long Baseline Interferometry (VLBI) can be used toconstruct images of the radio emission of active galactic nuclei on scalesof parsecs. Although BL Lacertac objects distinguished from othercore-dominated AGN primarily in the absence of strong optical lineemission - have "core-jet" structures similar to those seen in othertypes of compact extragalactic radio sources, their parsec-scale structureshows a number of distinctive features, such as the dominanceof transverse magnetic fields. This B-field structure may be due tothe presence of relativistic shocks: alternatively, it may reflect thedominance of the toroidal comportent of an underlying helical B field.The superluminal speeds observed in the VLBI jets of BL Lac objectsare somewhat lower than those in quasars, suggesting that the typicalLorentz factors for the jet flows are smaller.