Concerning the conversion from laboratory scale tests for chemical processes to successful commercial-size design, the following topics are discussed: residence time distributions (distribution functions, moments of the distribution, normalized distributions, and special distribution functions); direct applications (surge damping, first order reactions, other simple reactions, and complex reactions); measurement techniques (closed systems, and open systems); analytical distribution functions (piston flow and ideal mixing, tanks in series, laminar scaleup considerations (the almost delta distribution, laminar flow systems, and the almost exponential distribution); and complications and extensions (intermediate distributions, unmixed feed streams, and nonisothermal or nonhomogeneous reactors).