Cellulosomes are high molecular weight multienzyme cellulolytic complexes produced by Clostridium thermocellum and other anaerobic bacteria. They consist of a number of enzymes attached to a scaffolding protein, which contains a cellulose binding domain and several cohesin domains that interact with complementary dockerin domains of the catalytic subunits, integrating them into the complex. This organization concentrates many enzyme activities on the fibre surface, achieving remarkably efficient breakdown that inspires engineered biomass-conversion systems. Synthetic biologists now build artificial cellulosomes for tailored biomass breakdown.