A class of blazar. They are distant quasar-like objects with no spectral lines which are often highly variable (by up to a factor of 100 over a few months). They exhibit high intensity emission from radio to gamma ray wavelengths. An example is PKS 2155-304, one of the brightest X-ray emitter. Most theorists believe that observations of BL Lacs are detecting a jet of material moving at close to the speed of light towards the observer. However, BL Lacs may be caused by microlensing of individual stars in intervening galaxies (Ostriker and Vietri 1990).