Secondary ion mass spectrometry (SIMS) is a technique for chemical analysis and imaging of solid materials, with applications in many areas of science and technology. It involves bombarding a sample surface under high vacuum with energetic primary ions. The ejected secondary ions undergo mass-to-charge ratio (m/z) analysis and are detected. The resulting mass spectrum contains detailed surface chemical information with sub-monolayer sensitivity. Different experimental configurations provide chemically resolved depth distribution and 2D or 3D images. SIMS is complementary to other surface analysis techniques, such as X-ray photoelectron spectroscopy; chemical imaging techniques, for example, vibrational microspectroscopy methods such as Fourier transform infrared spectroscopy and Raman spectroscopy; and other mass spectrometry imaging techniques, including desorption electrospray ionization and matrix-assisted laser desorption ionization. Features of SIMS include high spatial resolution, high depth resolution and broad chemical sensitivity to all elements, isotopes and molecules up to several thousand mass units. This Primer describes the operating principles of SIMS and outlines how the instrument geometry and operational parameters enable different modes of operation and information to be obtained. Applications, including materials science, surface science, electronic devices, geosciences and life sciences, are explored, finishing with an outlook for the technique.