The scale-up of bubbling fluidized bed reactors to commercial size is a complex and troublesome endeavor. In particular, the proper scale-up of the hydrodynamics and chemical conversion inside the fluidized bed is subject to many eventualities and pitfalls which may drastically deteriorate the reactor's performance and economy. The main challenge of fluidized bed scaling is the scale dependence of many of the key reactor parameters. In this review, pathways to successful scale-up of fluidized bed reactors are presented. The most commonly described approach in open literature to achieve hydrodynamic and reactive similarity in two fluidized beds is to use sets of dimensionless numbers which have to be kept constant at both scales. These sets of dimensionless numbers have been modified and extended by several authors in order to have a more integral view on the diverse phenomena taking place in fluidized bed reactors. Successful scaling can be evaluated by various validation tools from classical time-series analysis to novel methods adapted from chaos theory. Over the past decades, the approach of scaling with dimensionless numbers has been applied to several fluidized beds in industry and research. While in older studies the scale-up is reportedly successful, in more recent studies this traditional scaling approach has repeatedly been questioned. Reasons for the emerging criticism are manifold. In particular the lacking degrees of freedom in the reactor design and the incompleteness of the scaling sets with respect to essential phenomena such as wall effects and particle-particle interactions are often criticized. Nonetheless, due to missing alternatives and as long as scale changes are small, traditional dimensionless scaling is still a viable route to successful fluidized bed reactor scaling. (C) 2011 Elsevier B.V. All rights reserved.