Flow nuclear magnetic resonance (NMR) spectroscopy is a powerful method for monitoring chemical reactions to improve their understanding and allow rational optimization. Flow NMR consists of making the reaction mixture flow through the NMR detector and can be tailored for the analysis of either batch or flow reactions. It provides quantitative measurements together with structural information over a broad range of conditions. The practice of flow NMR requires specialized hardware and know-how, some of which involves unusual spin effects, and requires customized methods for data collection and analysis. This Primer gives an overview of the features of flow NMR for chemical synthesis aimed at newcomers to the field. The reward of mastering flow NMR practice is illustrated through a selection of representative applications to mechanistic studies and reaction optimization in fields covering catalysis, polymer chemistry and autonomous synthesis. The range of flow NMR applications is continuously expanding thanks to new method developments that are described together with key challenges and limitations of the technique.