  Since the first TeV blazar Markarian (Mrk) 421 was detected in 1992, the
number of established TeV gamma-ray emitting BL Lac objects has grown to 6,
with redshifts ranging from 0.031 (Mrk 421) to 0.129 (H 1426+428). The
intensive study of these sources has had a major impact on our understanding of
the blazar phenomenon. The most notable observational results have been
extremely fast large amplitude flux and spectral variability on hour time
scales, and a pronounced X-ray - TeV gamma-ray flux correlation. In this paper
we discuss recent observational results and report on progress in their
theoretical interpretation.
