<p>Surface plasmon resonance (SPR) involves the photon-induced excitation of free electrons at the surface of a metal thin film, generating bound electromagnetic waves that are sensitive to refractive index variations of the medium in contact with the metal film. SPR enables the sensitive and label-free in situ detection of molecular interactions and film thickness measurements down to the angstrom level, making SPR a resource for biosensing, and the characterization of ultrathin and thin films and surface processes. This Primer introduces the general principles of the optical excitation and detection of propagative surface plasmons via conventional prism coupling, which is the most widely used configuration. We present the main steps of a typical SPR measurement and overview the fitting of SPR curves to quantify the thickness of an adsorbed film and its molecular surface coverage, as well as the analysis of sensorgrams in the context of binding kinetics and affinity constants. Some representative examples of the implementation of SPR in life and materials sciences are highlighted. We outline limitations of SPR sensing and issues with reproducibility, and discuss current challenges and new opportunities presented by machine learning, digital assays and SPR imaging.</p>