This talk will review our knowledge of extraterrestrial worlds. It will describe how our understanding of the interior structure and dynamics of icy moons has been revolutionized by the data collected by the Cassini mission which has been orbiting in the Saturn for about 12 years. The data that reveal the presence of deep oceans within Titan and Enceladus will be reviewed. The talk will address the potential habitability of these two fascinating moons of the solar system and describe some habitable niches.

As NASA and ESA are preparing their missions to Europa and Ganymede, respectively, this talk will describe their science objectives. It will discuss the similarities and the differences between these ocean worlds and their Saturn's cousins.

Finally, this talk will mention extra-solar ocean worlds as one class of planets that may be characterized in the future as thousands of exoplanets are being discovered.