  Uranus and Neptune form a distinct class of planets in our solar system.
Given this fact, and ubiquity of similar-mass planets in other planetary
systems, it is essential to understand their interior structure and
composition. However, there are more open questions regarding these planets
than answers. In this review we concentrate on the things we do not know about
the interiors of Uranus and Neptune with a focus on why the planets may be
different, rather than the same. We next summarize the knowledge about the
planets' internal structure and evolution. Finally, we identify the topics that
should be investigated further on the theoretical front as well as required
observations from space missions.
