Ever since the breakthroughs due to the work of Freedman on topological and Donaldson on smooth 4-manifolds in the early 1980s, it became clear how different the dimension 4 is from any other dimension. The combination of these two breakthrough produced examples of differing (exotic) smooth structures not only on closed topological 4-manifods, but also on R 4 . I will discuss some methods to construct and prove exoticness, some open questions, and some recent (and not so recent) results.