The nuclear boundary is formed by the nuclear envelope (NE), a double membrane system that establishes a selective barrier between the nucleoplasm and the cytoplasm. Its barrier characteristics are determined by nuclear pore complexes (NPCs); huge protein assemblies that guard nucleocytoplasmic compartmentalization. In this review, we discuss the biogenesis of NPCs during interphase of the cell cycle. This process requires a mechanistically enigmatic fusion step between the inner and the outer nuclear membrane. We focus on the principle of membrane pore formation in the NE and consider existing paradigms of other cellular membrane remodeling events. The emerging roles of transmembrane proteins and membrane-shaping factors in NPC biogenesis are discussed.