  The BL Lac object 3C 66A was the target of an extensive multiwavelength
monitoring campaign from July 2003 through April 2004, involving observations
throughout the electromagnetic spectrum. Radio, infrared, and optical
observations were carried out by the WEBT-ENIGMA collaboration. At higher
energies, 3C 66A was observed in X-rays (RXTE), and at very-high-energy (VHE)
gamma-rays (STACEE, VERITAS). In addition, the source has been observed with
the VLBA at 9 epochs throughout the period September 2003 - December 2004,
including 3 epochs contemporaneous with the core campaign.The WEBT campaign
revealed microvariability with flux changes of ~ 5 % on time scales as short as
\~ 2 hr. The source was in a relatively bright state, with several bright
flares on time scales of several days. The SED indicates a $\nu F_{\nu}$ peak
in the optical regime. A weak trend of optical spectral hysteresis with a trend
of spectral softening throughout both the rising and decaying phases, has been
found. The 3 - 10 keV X-ray flux of 3C 66A during the core campaign was
historically high and its spectrum very soft, indicating that the low-frequency
component of the broadband SED extends beyond ~ 10 keV. No significant X-ray
flux and/or spectral variability was detected. STACEE and Whipple observations
provided upper flux limits at > 150 GeV and > 390 GeV, respectively. The 22 and
43 GHz data from the 3 VLBA epochs between September 2003 and January 2004
indicate a smooth jet with only moderate internal structure. Evidence for
superluminal motion (8.5 +/- 5.6 h^{-1} c) was found in only one out of 6
components. The radial radio brightness profile suggests a magnetic field decay
$\propto r^{-1}$ and, thus, a predominantly perpendicular magnetic field.
