The direct access to the interior of the Sun has been possible, over the past 25 years, by successfully applying seismic techniques to detect and to characterize the whole spectrum of its oscillation eigenmodes. The unique and huge amount of observational data provided by earth-based networks and space-borne instruments, has allowed a high degree of refinement on present Solar Models as well as the development of robust inversion techniques. In addition, the sensitivity and correlation of acoustic solar eigenmodes characteristics (frequencies, power, linewidths, etc.) to solar activity (as measured by any of the many indicators) is providing further and additional insights on the so-called "Solar Activity Cycle". In this talk, a comprehensive review on the actual knowledge of the interior of the Sun and its relation with solar variability will be presented.