The influence of operational conditions, catalyst properties and reactor dimensions on the behavior of trickle bed reactors for heterogeneously catalyzed gas-liquid reactions is illustrated. The focus is on the description of the complex hydrodynamics and transport processes and their consideration in reactor modeling. In addition, aspects of process design based on laboratory experiments are highlighted and criteria for assessing the deviation from the ideal tubular reactor are discussed. To overcome mass transfer limitations, intensification concepts for trickle bed reactors are presented.