The dependence of photochemistry on excitation wavelength is not a recently observed phenomenon; nonetheless, it has, surprisingly enough, been largely ignored in the field. The reasons for this situation are not fully understood but might be related to a provisional extension of Kasha’s rule to photochemistry or perhaps to a difficulty in justifying the kind of short timescales implied in such photochemistry, which challenges the usually held view giving predominance to fast internal conversion and vibrational relaxation. Regardless of the reasons, it is still a matter of fact that a complete and satisfactory interpretation for experimentally proven wavelength-dependent photochemistry is not yet available, and the community endeavours to build a holistic understanding and a comprehensive view of the phenomenon. The present chapter compiles a non-exhaustive overview of the published data in the field, reporting on some of the most prominent features, issues and interpretations. Part of this chapter has been reused from Maafi [1] under an Open Access CC BY 4.0 license, https://creativecommons.org/licenses/by/4.0/ .