Isothermal titration calorimetry (ITC) has become the gold standard for studying molecular interactions in solution. Although it is increasingly being used in the soft matter and synthetic chemistry fields, ITC is most widely used for characterizing molecular interactions between ligands and macromolecules. This Primer starts by presenting the technique’s foundations and instrumentation, including a brief description of the standard assay, followed by a review of common applications. Further extensions and modifications of the technique are explored. These adaptations enable key features to be studied, such as cooperative effects associated with complex biological interactions and their regulation, alongside applications to other fields, including partition to membranes, kinetics and soft matter. Advantages and caveats in ITC are discussed, with a focus on best practices, instrument calibration, experimental design, data analysis and data reporting, as well as recent and future developments.