The promising potential applications, such as information processing and storage or molecular spintronics, of single-molecule magnets (SMMs) have spurred on the research of new, better SMMs. In this context, lanthanide ions have been seen as ideal candidates for new heterometallic transition metal-lanthanide SMMs. This perspective reviews 3d-4f SMMs up to 2014 and highlights the most significant advances and challenges of the field.