The iron-group transition-metal diselenides (MSe₂, M = Fe, Co, Ni) have recently gained tremendous interest in many areas. The MSe₂ compounds normally form similar pyrite or marcasite crystal structures, and, with a progressive increase in the metal 3d electrons, FeSe₂, CoSe₂, and NiSe₂ exhibit diverse and fascinating optical, magnetic, and electrical properties. In this talk, I will present our latest progress on the atomic layer deposition (ALD) of FeSe₂, CoSe₂, and NiSe₂. These ALD processes employ the corresponding metal amidinates as the metal precursors and diethyldiselenide (DEDSe) as the selenium precursor, together with Ar/H₂ plasma for DEDSe activation. All of the ALD processes are able to grow pure, smooth, and crystalline MSe₂ (M = Fe, Co, Ni) films, and the MSe₂ films can also be uniformly deposited into 10:1 high-aspect-ratio microtrenches with excellent conformality, which thereby shows the great promise for conformal coating on 3D complex topologies in general. In situ ALD mechanism investigation further reveals that the efficient dissociation of DEDSe by plasma is key to the success of these ALD processes.