The three-dimensional atom-probe is the only analytical microscope able to map out the 3D distribution of chemical species at the atomic scale. Combined with a Field Ion Microscope, 3D Atom Probe is particularly well adapted to the investigation of the fine-scale features of the microstructure of metallic alloys. 3D atom probe makes it possible to measure quantitatively the composition in a small region that can be selected arbitrarily in the reconstructed volume. This way, the composition of small precipitates or that of the surrounding solid solution or even that of a thin layer close to interfaces can be measured. The potential of this new generation of analytical microscope is illustrated through examples (segregation of boron to planar faults in FeAl and superalloys, precipitation and ordering...).