  In this talk we briefly describe some basic elements of chiral perturbation
theory, $\chi PT$, and how the implementation of unitarity and other novel
elements lead to a better expansion of the $T$ matrix for meson meson and meson
baryon interactions. Applications are then done to the $ \pi \pi $ interaction
in nuclear matter in the scalar and vector channels, antikaons in nuclei and
$K^-$ atoms, and how the $\phi$ meson properties are changed in a nuclear
medium.
