Many thermophilic bacteria are efficient biomass degraders (producing polysaccharide degrading enzymes and utilizing a great variety of substrates, e.g. lignocellulosic polymers, pentoses, hexoses, as well sugar acids, and sugar alcohols). This makes them interesting organisms as potential cell factories in a circular bioeconomy. Lignocellulosic and marine macroalgal biomasses are regarded as sustainable biorefinery feedstocks for the production of energy carriers and platform and specialty chemicals, thereby meeting impending fossil fuel shortage and counteracting accumulation of greenhouse gasses. However, progress in using thermophilic bacteria that utilize these feedstocks as carbon sources has been hampered by the lack of suitable engineering tools to improve the production profiles of interesting target metabolites as specific synthetic production pathways need to be inserted/modified or existing pathways optimized by metabolic engineering. In this chapter, we review the progress on the use of thermophilic bacteria in metabolic engineering and the available engineering tools and give examples of species for which successful engineering has been accomplished. Today, the majority of thermophilic bacteria targeted for production of compounds of industrial interest by metabolic engineering belong to the phylum Firmicutes (e.g. Thermoanaerobacterium, Caldocellulosiruptor, Geobacillus, and Bacillus), taking advantage of anaerobic catabolic pathways producing organic acids and alcohols. However, there are additional and aerobic species gaining interest concerning biomass degradation and the ability of carbon dioxide fixation as well as production of molecules of interest, and some examples of this are also given.