The phenomenon whereby the → photodissociation transitions of a molecule in interstellar clouds become → optically thick, so that the molecule in question is "shielded" by other molecules against dissociating stellar → far-ultraviolet (FUV) photons. In the case of → molecular hydrogen (H2), when the → column density exceeds 1014 cm-2, the UV absorption bands become optically thick, and H2 undergoes self-shielding. More specifically, all of the photons that could lead to UV photodissociation are absorbed by H2 in the outer layers of the cloud, hence protecting the H2 within the cloud. Self-shielding occurs in → diffuse interstellar clouds exposed to the interstellar → radiation field or in → molecular clouds in proximity to sources of UV photons. Dust can also absorb UV photons, further limiting the photodissociation, but it dominates only when the local UV radiation field is unusually intense relative to the density of the cloud.