Physical chemistry lies at the heart of one of chemistry's principal applications and achievements: the identification of the substances present in a sample and the determination of their abundances and structures. ‘Investigating matter’ considers how the laser and computer have elaborated and refined classical techniques. Various forms of investigative technique are explained including absorption spectroscopy and nuclear magnetic resonance. Mass spectrometry is widely used in organic chemistry to help identify compounds and photoelectron spectroscopy is used both to explore the energies with which electrons are bound inside molecules and to identify species on surfaces. The study of surfaces has been transformed by scanning tunnelling microscopy, which relies on quantum mechanics.