The pathophysiology of heart failure is characterized by abnormalities not only of the cardiomyocytes, but also by expansion of the extracellular space. Evolving recognition of these changes, and their important functional consequences, has prompted a search for accurate characterization of the myocardial interstitium. Previously, this has only been possible using histologic analysis; however, T₁ mapping with cardiac magnetic resonance imaging has recently provided a noninvasive method to measure expansion of the myocardial interstitium. Literature over the past 6 years suggests T₁ mapping could potentially provide crucial information for diagnosis, prognostication and for optimizing therapeutic targeting. However, the ideal methodology for both image acquisition and analysis remains an unresolved issue. Consequently, variations in methodology have complicated interpretation of the data, and limit comparisons between studies. This review provides a summary of important initial validation papers, and subsequent diagnostic, mechanistic and longitudinal studies of T₁ mapping in heart failure. In addition, variations in methodology are discussed; highlighting current challenges facing scientists and clinicians interested in this evolving field.