The Bohigas-Giannoni-Schmit conjecture stating that the statistical sp
ectral properties of systems which are chaotic in their classical limi
t coincide with random matrix theory (RMT) is proved. A new semiclassi
cal field theory for individual chaotic systems is constructed in the
framework of a nonlinear sigma model. The low lying modes are shown to
be associated with the Perron-Frobenius (PF) spectrum of the underlyi
ng irreversible classical dynamics. It is shown that the existence of
a gap in the PF spectrum results in RMT behavior. Moreover, our formal
ism offers a way of calculating system specific corrections beyond RMT
.