The myocardial interstitial or extracellular space exists as a complex and dynamic environment, vital for normal cardiac structure and function. The physiological pathways for normal control of collagen turnover, and the pathological development of fibrosis are beginning to be understood, as are their relationships to cardiac remodelling and adverse outcomes. Emerging non-invasive imaging techniques (echocardiography, cardiovascular magnetic resonance, positron emission tomography) may allow a clearer understanding and measurement of these processes in vivo. Preliminary results are exciting, spanning valvular and congenital heart disease, cardiomyopathy and rarer diseases such as amyloid. In this review, such developments and research directions are explored, including the rapid developments in cardiovascular magnetic resonance T1 mapping and its use with contrast to derive extracellular volume. The authors present a state-of-the-art assessment of the strengths and weaknesses of each modality, and distil a framework to equip the reader with an understanding of the technical issues useful for the interpretation of emerging clinical studies.