Stem cells have the potential to proliferate, self-renew and differentiate into various organ-specific cell types. They can be of either human or non-human origin, and they can be classified according to (a) their developmental stage, as embryonic, fetal or adult; (b) their potential to differentiate into one or more specific types of mature cell, as totipotent, pluripotent or multipotent; and (c) the tissue of origin as haematopoietic, mesenchymal, skeletal or neural. Embryonic stem cells are totipotent and have the potential to become all specialized cell types, whereas adult stem cells are undifferentiated cells found in differentiated tissue with the potential to renew themselves and to differentiate to yield organ-specific tissues. The remarkable proliferative and differentiation capacity of stem cells has raised the expectation of an almost unlimited supply of specific cell types. It has been therefore hypothesized that stem cells could differentiate into cardiomyocytes. In their review in the Journal of Nuclear Cardiology, Chang et al. [1] from the Department of Medicine, Divisions of Cardiology and Radiology, Molecular Imaging Program at Stanford, Stanford University School of Medicine, California discuss the use of stem cell therapy and imaging in the context of cardiology and point out that stem cell therapy is emerging as a promising approach to treat heart diseases. Considerable evidence from experimental studies and initial clinical trials suggests that stem cell transplantation promotes systolic function and prevents ventricular remodelling. Thus stem cells are candidates for novel therapeutic strategies for patients with different heart diseases most commonly caused by myocardial infarction. However, the specific mechanisms by which stem cells improve heart function remain largely unknown. In addition, interpreting the long-term effects of stem cell therapy is difficult because of the limitations of conventional techniques. Recent randomized studies of cell therapy for heart disease [2–7] constitute examples of how rapidly this field is developing and they also uncover many critical issues that have yet to be addressed [8]. The fundamental concepts related to the use of stem cells in cardiovascular disease have also been summarized by Penn and Mal from the Department of Cardiovascular Medicine, The Cleveland Clinic Foundation, in their recent paper [9] and by van der Bogt et al. from the Molecular Imaging Program at Stanford, Laboratory of Cardiothoracic Transplantation, Stanford University School of Medicine, California, and the Department of Surgery, Leiden University School of Medicine, The Netherlands. Indeed, the variety of available cell types and methods for cell delivery to the myocardium raises questions about their use for therapy [10]. In his paper in Heart Failure Reviews [11], Bengel, from the Division of Nuclear Medicine, Russell H Morgan Department of Radiology and Radiological Sciences, Johns Hopkins University Medical Institutions, remarks that, despite an increasing body of experimental and human data, this innovative and promising therapeutic approach for heart failure still presents a substantial challenge for basic scientists and clinical researchers. In particular, two major concerns are the ability of undifferentiated stem cells to form teratomas and the possibility of a provoked immune reaction after transplantation of stem cells into a new host. In their paper, van der Bogt and colleagues discuss the Eur J Nucl Med Mol Imaging (2007) 34:422–425 DOI 10.1007/s00259-007-0369-6