A scale-up of crystallization process is a very important yet not solved issue. In this paper the theoretical reasoning considering this problem is presented. The classical method involving preservation of geometrical similarity is discussed. The alternative way to success leading through combination of hydraulic and kinetic requirements is presented. This approach involves departure from geometrical similarity together with primary circulation time (PCT) as a scale-up criterion. (C) 2015 Elsevier B.V. All rights reserved.