The bright Seyfert 1 galaxy NGC 4151 was monitored intensively by IUE, ROSAT, ASCA, CGRO, and ground-based telescopes for 10 days in 1993 December. The source, which was near its peak historical brightness, showed strong, correlated variability at X-ray, ultraviolet, and optical wavelengths (see Fig. la). The strongest variations were seen in medium energy (~1.5keV) X-rays, with a normalized variability amplitude (NVA) of 24%. Weaker (NVA = 6%) variations, uncorrelated with those at lower energies, were seen at soft γ-ray energies of ~100keV. No significant variability was seen in softer (0.1–1 keV) X-ray bands. In the ultraviolet/optical regime, the NVA decreased within increasing wavelength, from 9% to 1% between 1275 Å and 6900 Å.