Homogeneous Brønsted acid catalysts such as H2SO4 and HCl are used for the production of industrially important chemicals. However, their use requires significant energy costs for separation, reuse, and treatment of salt wastes. Alternatively, heterogeneous Brønsted acid catalysts are promising candidates that can decrease the environmental impact associated with chemical production. In this review, we highlight amorphous carbon bearing SO3H groups as an insoluble Brønsted acid available for various acid-catalyzed reactions.