Isothermal titration calorimetry (ITC) is emerging as a premier tool for characterizing interactions of biological macromolecules by virtue of its general applicability, high precision, and high molecular-information content. ITC measures binding from the universal heat / enthalpy change associated with binding interactions. Binding equilibria are measured in solution phase, without need for covalent- or radio-labeling, and attainment of equilibrium can be verified during the titration. Because the data are rich in thermodynamic information, ITC reveals aspects about the functional chemistry of an interaction that are invisible to conventional affinity measurements. We will review at an introductory level the underlying principles of ITC measurements on biological macromolecule-ligand interactions, pointing out strengths and limitations. We will also discuss examples where ITC was used to characterize: 1) protein-protein interactions involving the extracellular domain of the human T-cell co-receptor, CD4, and 2) a receptor extracellular domain-peptide interaction. In these examples ITC was used to characterize various aspects of the functional chemistry of the protein-ligand interactions including determination of equilibrium affinity constants, the number of protons coupled to binding, and the binding thermodynamics. Evaluation of the binding thermodynamics provides insight into the extent of conformational changes which are predicted to accompany ligand binding.