The nuclear pore complex (NPC) undoubtedly assumes a central role in the communication between nucleus and cytoplasm. It controls both the export of mature nuclei acids from the nucleus and the import of large proteins, such as transcription factors, into the nucleus. Light microscopy enables the visualization of single NPCs as bright diffraction-limited spots. Combination of image analysis methods and distribution analysis allows the assessment and characterization of three-dimensional arrangement on the nuclear envelope. This is made possible by two features of the NPC system: (1) the large diameter of the pore complexes, which nearly approach the lateral optical resolution and (2) a relatively low pore density on the nuclear envelope surface. This was determined to be 4–5/μm 2 for 3T3 cells. Because many cell types have higher NPC densities (up to more than 50/μm 2 ), a detailed analysis of the pore distribution can be applied only in a limited number of systems.