Controlling the wettability of solid surfaces is an important issue that has aroused the increasing interest from both fundamental and practical perspective by tailoring surface morphology and surface chemical compositions. The underlying theories for interpreting wetting phenomena still mainly focus on the Young's equation, the Wenzel equation, and the Cassie-Baxter equation, despite the fact that the wetting phenomena have been studied over the past decades. While there are a lot of experimental studies on wettability of surface roughness, there is still a lack of a thorough analysis of the contributions of micro and nano-scale roughness to wettability behavior despite interesting features these surfaces have. In this article, the basic theories and their applicabilities are addressed in detail, and the mutual transition between Wenzel state and Cassie-Baxter state is described from different viewpoints in general, and from single-scale and dual-scale point of view in particular. The design concept of geometrical model with stable superhydrophobicity is also described, which is based on the typical theories about wettability. Finally, some promising breakthroughs in the theoretical progress are proposed.