The Tomographic Atom-Probe is a new generation of instrument which provides three-dimensional maps of chemical heterogeneities in a metallic material on a subnanometric scale. This technique was applied to the investigation of precipitation and grain-boundary (GB) segregation phenomena in Astroloy nickel base superalloys. 3D images as provided by the tomographic atom-probe reveal the serrated geometry of GB's on a very fine scale and the presence of a strong segregation of both boron and molybdenum at grain-boundaries. Slight carbon enrichment is also detected. Considerable chromium segregation is exhibited at gamma'-gamma' grain-boundaries. Ail these segregants are distributed in a continuous manner along the boundary over a width close to 0.5 nm. Long range order in the gamma'ordered phase is shown to be preserved up to the grain boundary.