Transport between nucleus and cytoplasm is exclusively mediated by nuclear pore complexes (NPCs) embedded in the nuclear envelope. The NPC is an enormously elaborate protein assembly, reflecting its ability to multitask by simultaneously regulating the trafficking of a diverse spectrum of substrates, ranging from microRNAs to assembled ribosomal subunits. The complexity and sheer size of the NPC have hampered efforts to elucidate its molecular architecture. However, recent studies using a battery of complementary techniques have significantly enhanced our understanding of the NPC structure. The picture of a highly dynamic and modular machine is emerging.