  We provide an overview of the successive steps that made possible to obtain
increasingly accurate excitation energies with computational chemistry tools,
eventually leading to chemically accurate vertical transition energies for
small- and medium-size molecules. First, we describe the evolution of
\textit{ab initio} methods employed to define benchmark values, with originally
Roos' CASPT2 method, then the CC3 method as in the renowned Thiel set, and more
recently the resurgence of selected configuration interaction methods. The
latter method has been able to deliver consistently, for both single and double
excitations, highly accurate excitation energies for small molecules, as well
as medium-size molecules with compact basis sets. Second, we describe how these
high-level methods and the creation of representative benchmark sets of
excitation energies have allowed to assess fairly and accurately the
performance of computationally lighter methods. We conclude by discussing the
future theoretical and technological developments in the field.
