Atom probe field ion microscopy has revealed that excessive copper levels in pressure vessel steels result in the formation of ultrafine copper-enriched clusters and precipitates during in-service neutron irradiation. In addition to these important clusters and precipitates, this technique has revealed many other fine-scaled microstructural features in these steels. These microstructural features may be divided into two broad categories; 1) intragranular precipitates and clusters and 2) interfacial segregation. The precipitates and clusters include spherical, rod-shaped and disk-shaped molybdenum and vanadium carbides, disk-shaped molybdenum nitrides, and spherical iron oxides. The type of precipitate is dependent on the metallic additions that are made to the various types of steels used in the pressure vessel. Some of these features are present in approximately the same number density and size as the copper clusters and precipitates and will therefore make a contribution to the mechanical properties. The characterization of grain boundaries and precipitate/matrix interfaces has revealed a complex pattern of segregation of various solutes including phosphorus, nickel and manganese.