  We present multi-wavelength observations of the very high energy (VHE, E >250
GeV) gamma-ray sources Mrk 421 and Mrk 501. The VHE data presented here were
taken with the Whipple Observatory 10m gamma-ray telescope. In the Mrk 421
campaign, conducted in 1995 April-May, correlations were observed between VHE
gamma-ray, X-ray, extreme ultraviolet (EUV) and possibly R-band emission. The
X-rays and gamma rays vary together with similar flux amplitude while the EUV
photons lag the X-rays and gamma rays by ~1 day and have a lower variability
amplitude. In the Mrk 501 campaign, conducted in 1997 April, a correlation was
also observed between the VHE gamma rays and X-rays with no delays apparent.
Here, the VHE gamma-ray variability is larger than the X-rays. The observations
also indicate that the synchrotron spectrum extends to 100 keV before cutting
off. For Mrk 421, the X-ray/VHE gamma-ray correlation indicates that delta > 38
and B > 0.03G. For Mrk 501, the correlation indicates a more modest delta > 2
and B > 0.09G.
