Medium-chain fatty acids (MCFAs) and their derivatives have important applications in the energy and chemical industries. The sustainable production of MCFAs from renewable biomass resources using microbial fermentation has become promising. A particular set of anaerobic pathways known as chain elongation can occur under specific conditions producing MCFAs with a higher value than biogas and broader applicability. MCFAs formation occurs through a carboxylic acid chain elongation process, which mainly uses reverse beta-oxidation of acetic acid, n-butyric acid, and caproic acid with an electron donor (ethanol, lactic acid, methanol, and hydrogen). The MCFAs fermentation by chain elongation is mainly composed of caproic acid (C6), heptylic acid (C7), and caprylic acid (C8). Therefore, we have summarized the current knowledge on the mechanism of chain elongation, a substrate for MCFAs production, microbial strains for MCFAs production, environmental factors affect on MCFAs production, process development for MCFAs production, improvement methods for MCFAs production and discussed the current state of the art on the use of complex organic feedstock and reviewed key operational parameters and their interactions.