  Models with three active neutrinos and one sterile neutrino can naturally
account for maximal oscillations of atmospheric neutrinos, explain the solar
neutrino deficit, and accommodate the results of the LSND experiment. The
models predict either $\nu_e\to\nu_\tau$ or $\nu_e\to\nu_s$ oscillations in
long-baseline experiments with the atmospheric $\delta m^2$ scale and amplitude
determined by the LSND oscillations.
