Book excerpt: A combination of biochemical, biophysical, immunochemical and ultrastructural techniques, followed by molecular biological verification, led to the definition and proof of the cellulosome concept. The birth of the discrete, multi-enzyme cellulosome complex was thus documented. Today, cellulosomes have been confirmed in numerous but not all cellulolytic bacteria. The cellulosome-producing strains exhibit surprising diversity in the composition and architecture of the component parts.