Applying a high voltage to a liquid passing through a capillary disintegrates the liquid meniscus to generate a fine spray of charged droplets. As the droplets evaporate, polar molecules dissolved in the liquid are ionized and brought into the gas phase. This phenomenon is commonly known as electrospray ionization (ESI). The gaseous ions can be analysed using mass spectrometry (MS) to determine their molecular masses and chemical structures. Polar analytes — including low-molecular-weight species such as amino acids, peptides, nucleotides and synthetic chemicals; macromolecules, for example, synthetic and natural polymers; and nanoscale particles, for instance, viruses — are ionizable using ESI. In the past three decades, ESI-MS has evolved into a powerful instrumental approach useful to (bio)chemists in a wide range of applications. Several different variants have been developed to extend its functionality. For obtaining chemical and structural information on macromolecules, ESI-MS is complementary to fluorescence spectroscopy, nuclear magnetic resonance spectroscopy, X-ray crystallography and cryo-electron microscopy. This Primer discusses the ESI setup, mechanism and variants. It further introduces sample and instrument-related factors that affect the ionization process, applications and current research, as well as giving a future perspective.