X2P techniques combine the advantages of core-electron transitions with those of standard 2P techniques: the quadratic dependence on intensity endows two-photon techniques with better spatial resolution, whereas core-level transitions afford elemental and orbital sensitivity to the local environment and oxidation states. On the downside, in the high-intensity regime, X2PA is competing against single-photon and sequential multiphoton processes, such as core photoionization, stimulated emission, and scattering, which complicates the interpretation of the experimental observations. So far, the published experimental papers have focused on transition metals. Competing processes such as photoionization and molecular fragmentation have largely hampered experiments on small molecules.