We have combined structural, biochemical and recombinant DNA methods to explore molecular interactions involved in nuclear envelope assembly dynamics and nucleocytoplasmic transport. Electron microscopy has established the overall architecture of the envelope and the relationship between nuclear pores, lamina fibres and pore-connecting fibrils. The lamin proteins that constitute the lamina resemble intermediate filament proteins, and assemble and disassemble during mitosis in response to phosphorylation. Lamins have been expressed in E. coli to facilitate structural investigations and the exploration of interaction sites with other envelope components. Disruption of envelopes has shown that nuclear pores are constructed from a central cylinder with cytoplasmic and nucleoplasmic rings. Examination of envelopes transporting gold-labelled nucleoplasmin has indicated that the transport pathway is complex and probably involves ring components in addition to the central cylinder. Molecular motors may be involved in changes in pore shape to enable transport and in the translocation mechanism.