  Wavefunctions in chaotic and disordered quantum billiards are studied
experimentally using thin microwave cavities. The chaotic wavefunctions display
universal density distributions and density auto-correlations in agreement with
expressions derived fro m a 0-D nonlinear $\sigma $-model of supersymmetry,
which coincides with Random Matrix Theory. In contrast, disordered
wavefunctions show deviations from this universal behavior due to Anderson
localization. A systematic behavior of the distribution function is studied as
a function of the localization length, and can be understood in the framework
of a 1-D version of the nonlinear $\sigma $-model.
