Combining the WKB (Wentzel-Kramers-Brillouin) method and the Borel resummation method, A. Voros initiated the exact WKB analysis. Now the method is known as a powerful tool to study global properties (monodromy / Stokes structures) of solutions of differential equations with a large parameter. The aim of this talk is to give an introduction to this theory. I'll also explain some connections to other topics such as cluster algebras, spectral networks, topological recursion, and so on.