The 3D atom probe (3DAP or atom probe tomography) is the only nanoanalytical microscope able to provide 3D atomic-scale images. The role of this instrument in the investigation of the microstructure of metallic alloys is illustrated through various examples. Thanks to the ultimate spatial resolution of 3DAP, a Cottrell atmosphere was imaged for the first time in 3D and at the atomic scale. Reconstructed images have dimensions close to that of Monte Carlo simulations (MCS), as a result real experiment and numerical experiment can be directly confronted so that to validate models and MCS parameters. This will be illustrated through the investigation of the kinetics of precipitation and ordering in a model NiCrAl superalloy. Both approaches indicate that phase separation starts through a classical nucleation and growth process rapidly followed by a corasening regime. \textcopyright 2005 The Physical Society of the Republic of China.