In many fields of science and technology, the chemistry of materials at the atomic scale is essential to the large-scale behaviour. In extreme environments, such as the high temperatures and stresses of a modern aerospace engine or the heat and irradiation of the walls of a nuclear reactor, even small changes in local composition can result in embrittlement, corrosion and cracking. Similarly in semiconductors, even a few thousand atoms in the wrong place can result in failure of a transistor or similar device. Atom probe tomography (APT) is becoming a vital tool in analysing the chemical behaviour of materials at this scale. Using an advanced ultra-high vacuum instrument, with specimens cooled to 20-80K, a small needle of the material is characterised atom by atom. An electric field is established between the specimen and the detector, just below the threshold required to remove an atom. A further pulse of either higher voltage or laser energy is applied to field evaporate individual atoms. The detector is position sensitive, and the time between the pulse and the detector event gives the time-of-flight and hence the mass/charge ratio of the detected ion. Using this method, specimens can be reconstructed into a 3-dimensional, chemically-sensitive dataset of anywhere up to hundreds of millions of atoms. With an analytical sensitivity up to 1ppm and resolutions of 0.3nm achievable in all three dimensions, this allows the chemistry of the material to be mapped to incredibly high precision. This webinar explains the fundamentals of the technique, and outlines a number of the applications of atom probe tomography being undertaken at the UK National APT facility at the University of Oxford. Some of the work includes characterisation of the tips of stress corrosion cracks in steels, clustering of minor elements in steels, studying the reaction of catalysts and other materials to corrosion and oxidation, as well as the distribution of dopants in GaN quantum wells. This webinar aims to demonstrate why APT is a powerful technique that is growing in influence and importance to many materials science applications.