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