A new class of giant resonances in nuclei is discussed, i.e. giant res
onances built on other giant resonances. These resonances are observed
with very large cross sections in relativistic heavy ion collisions.
A great experimental and theoretical effort is underway to understand
the reaction mechanism which leads to the excitation of these states i
n nuclei, as well as the better microscopic understanding of their pro
perties, for example, strength, energy centroids, widths and anharmoni
cities.