  We discuss the time dependent interferences between $K_L$ and $K_S$ in the
decays in $3\pi$ and $\pi\pi\gamma$, to be studied at interferometry machines
such as the $\phi$-factory and LEAR. We emphasize the possibilities and the
advantages of using interferences, in comparison with width measurements, to
obtain information both on $CP$ conserving and $CP$ violating amplitudes.
Comparison with present data and suggestions for future experiments are made.
