There has been a resurgence of interest for the field of cardiac metabolism catalysed by the increased need for new therapeutic targets for patients with heart failure. Traditionally, the focus of research in this area has been on the impact of substrate selection – carbohydrates vs. fatty acids – for mitochondrial oxidative energy metabolism. The use of recently emerging metabolomic technologies – which aim at systematically measure all low-molecular weight compounds within a biological system – has provided some novel insight into the global metabolic perturbations prevailing in several cardiovascular diseases. Acylcarnitines (ACs) are among metabolites that have been the focus of many recent metabolomics-based studies, particularly in human heart failure. Profiling of circulating ACs has commonly been used for diagnosis of inborn errors of metabolism, particularly mitochondrial fatty acid oxidation (FA) defects. Beyond being proxies of fatty acid metabolic dysregulation, ACs – primarily long-chain ACs (LCACs) – are, however, increasingly being recognized as “actors”, modulating cell functions, as well as being linked to adverse cardiac events such as arrhythmias. This presentation aims to provide an overview of metabolic pathways generating ACs and of studies reporting elevated circulating levels of ACs, particularly LCACs, in human heart failure. It will also discuss proposed molecular mechanisms for the potential adverse effects of LCACs on cardiac function.