Extruded monolith substrates are widely used in automotive and stationary emission control reactors such as selective catalytic reduction (SCR) units. Monoliths are increasingly being used, developed, and evaluated as catalyst supports in many new reactor applications such as chemical and refining processes, catalytic combustion, ozone abatement, and others. This paper gives a general overview of monolith fabrication, characteristics and typical use. Several commercial product applications and new developments for use of monolith reactors in automotive, stationary and chemical industry are discussed.