The nuclear envelope is composed of the nuclear lamina, nuclear pore complexes and nuclear membranes. The outer nuclear membrane is very similar to the rough endoplasmic reticulum. The pore membranes contain unique integral proteins and are associated with nuclear pore complexes. The inner nuclear membrane is associated with heterochromatin and the nuclear lamina, a meshwork of intermediate filament proteins called lamins. In humans, lamins are encoded by three genetic loci, LMNA, LMNB1 and LMNB2. Mutations in LMNA cause a spectrum of inherited diseases, including autosomal dominant Emery-Dreifuss muscular dystrophy and related striated muscle disorders, partial lipodystrophies, a peripheral neuropathy and progeria syndromes. Eighty or more transmembrane proteins may reside primarily in the inner nuclear membrane but only several have been fairly well characterized. These include emerin, which is mutated in X-linked Emery-Dreifuss muscular dystrophy, LAP2, MAN1 and LBR. LBR binds to B-type lamins and chromatin proteins and shares sequence similarities with sterol reductases. Heterozygous mutations in LBR cause Pelger-Huët anomaly, characterized by morphologically abnormal neutrophil nuclei, and homozygous mutations cause HEM/Greenberg skeletal dysplasia, characterized by developmental abnormalities and 3 beta-hydroxysterol-epsilon-14-reductase deficiency. Further studies of nuclear envelope proteins may uncover additional unsuspected relationships to human disease.