A Continuous Stirred Tank Reactor (CSTR) is a reaction vessel in which reagents, reactants and often solvents flow into the reactor while the product(s) of the reaction concurrently exit(s) the vessel. In this manner, the tank reactor is considered to be a valuable tool for continuous chemical processing.

CSTR reactors have effective mixing and perform under steady-state with uniform properties. Ideally, the output composition is identical to composition of the material inside the reactor, which is a function of residence time and reaction rate.

In situations where a reaction is too slow or when two immiscible or viscous liquids are present requiring a high agitation rate, several Continuous Stirred Tank Reactors (CSTRs) may be connected together forming a cascade.

A Continuous Stirred Tank Reactor (CSTR) assumes ideal mixing and thus, is the complete opposite of a Plug Flow Reactor (PFR).