---
abstract: 'We report on the results of a three-year program of coordinated X-ray and optical monitoring of the narrow-line Seyfert 1 galaxy NGC 4051. The rapid continuum variations observed in the X-ray spectra are not detected in the optical, although the [*time-averaged*]{} X-ray and optical continuum fluxes are well-correlated. Variations in the flux of the broad  line are found to lag behind the optical continuum variations by 6 days (with an uncertainty of 2–3 days), and combining this with the line width yields a virial mass estimate of $\sim1.1 \times 10^6$, at the very low end of the distribution of AGN masses measured by line reverberation. Strong variability of 686 is also detected, and the response time measured is similar to that of , but with a much larger uncertainty. The 686 line is almost five times broader than , and it is strongly blueward asymmetric, as are the high-ionization UV lines recorded in archival spectra of NGC 4051. The data are consistent with the Balmer lines arising in a low to moderate inclination disk-like configuration, and the high-ionization lines arising in an outflowing wind, of which we observe preferentially the near side. Previous observations of the narrow-line region 