Abstract: Thermoelectrochemical cells (also known as thermogalvanic cells or thermocells) are electrochemical devices that convert thermal energy to electrical energy via entropically driven redox reactions. These devices have gained increasing attention this century as they have the possibility of valorising otherwise wasted (heat) energy to useful (electrical) energy with no moving parts, no greenhouse gas emissions, and using sustainably sourced elements such as iron (Fe). Liquid thermocells suffer from several issues including electrolyte leakage, lower ‘observed’ temperature gradients than those applied and poor mechanical properties. Towards applications such as body heat harvesting — where thermal energy sources are dynamic — these disadvantages can become significant. Gelled electrolyte thermocells have been developed as these are self-contained systems that achieve higher temperature gradients across the thermocell and have mechanical properties that allow the ability to stretch, bend, and twist. This makes gelled thermocells compatible with many of the proposed applications of these devices. However, compared to liquid electrolyte thermocells, gelled electrolyte thermocells typically achieve significantly lower performance, mainly due to frustrated ion transport in the denser matrix, reducing the generation of current, which also leads to reductions in power output over time. This review provides an overview of the current state of gelled electrolyte thermocells and compares them to their liquid counterparts.