  We show that a {\em vibrational instability} of the spectrum of weakly
interacting quasi-local harmonic modes creates the maximum in the inelastic
scattering intensity in glasses, the Boson peak. The instability, limited by
anharmonicity, causes a complete reconstruction of the vibrational density of
states (DOS) below some frequency $\omega_c$, proportional to the strength of
interaction. The DOS of the new {\em harmonic modes} is independent of the
actual value of the anharmonicity. It is a universal function of frequency
depending on a single parameter -- the Boson peak frequency, $\omega_b$ which
is a function of interaction strength. The excess of the DOS over the Debye
value is $\propto\omega^4$ at low frequencies and linear in $\omega$ in the
interval $\omega_b \ll \omega \ll \omega_c$. Our results are in an excellent
agreement with recent experimental studies.
