What has helioseismology taught us about the Sun?

Helioseismology has revolutionized the study of the Sun. The analysis of helioseismic data have lead us to build a detailed picture of the solar interior and have led to interesting results about the basic properties of matter (as in the case of the solar neutrino problem). The data are now being used to study how the Sun changes with change of solar activity. We review some highlights of what we have learned so far from helioseismology. We also discuss the recent controversy over the solar heavy-element abundances. Helioseismic indicators point to a high solar metallicity.