  The non-thermal spectra of blazars, observed from radio to GeV/TeV
gamma-rays, reveal two pronounced components, both produced by radiation by
energetic particles. One peaks in the IR - to soft X-ray band, radiating via
the synchrotron process; the other, peaking in the high-energy gamma-rays, is
produced by the Compton process. These spectra -- and, in particular, the \asca
data -- suggest that the origin of the seed photons for Comptonization is
diverse. In the High-energy peaked BL Lac objects (HBLs), the dominant seed
photons for Comptonization appear to be the synchrotron photons internal to the
jet (SSC process). In the quasar-hosted blazars (QHBs), on the other hand, the
X-ray band emission is still dominated by the SSC process, while the MeV to GeV
range is produced by Comptonization of external photons such as the emission
line light. In the context of this three-component model, we derive the
magnetic field of 0.1 - 1 Gauss for all classes of blazars. Lorentz factors
gamma_{peak} of electrons radiating at each peak of the nuFnu spectra are
estimated to be ~10^{5}$ for HBLs; this is much higher than ~10^{3}$ for QHBs.
This difference is consistent with the fact that the four sources that are
known to emit TeV gamma-rays (TeV blazars) are all classified as HBLs. Among
the TeV blazars, Mkn 421 is one of the brightest and most variable emitters
from ultraviolet (eV) to hard gamma-ray (TeV) energies.The multi-frequency
observations including TeV energy band provide the best opportunity to
understand high-energy emission from blazar jets. In this paper, we discuss
results of multi-frequency analysis and review the results of intensive
campaigns for Mkn 421 from 1994 to 1998
