Pd(I) is the least understood oxidation-state for palladium species. However, in recent years, Pd(I) species have received increasing attention for their roles in Pd-catalyzed organic transformations. A number of dimeric Pd(I) compounds have proved effective catalysts for cross-coupling reactions. As a notable example, the Pd(I) dimer [(P t Bu3)PdBr]2 can function as the reservoir of the highly reactive 12-electron Pd(0) species and has been applied as precatalysts for an array of C-C and C-N bond cross-coupling reactions. Mononuclear Pd(I) species are invoked as putative intermediates in Pd-catalyzed halogen-atom-transfer reactions, carbonylation reactions, cross-couplings, as well as light-driven Pd-catalyzed reactions. In these catalytic reactions, single-electron redox reactions between Pd(0) species and organic halides and lightdriven decomposition of Pd(II) species are proposed to yield Pd(I) intermediates. The transient nature of these mononuclear Pd(I) intermediates, however, prevents isolation and detailed characterization.