This paper gives a review of recent results from important publications on two-phase flow in trickle-bed and monolithic reactors. Flow regimes encountered in these reactors are discussed and evaluated concerning mass transfer performance. In particular a model is presented to calculate the velocity fields and phase distributions for two-phase flow of vapor and liquid in packings and monoliths. Using the model even flow instabilities can be detected numerically. Therefore the packing is subdivided into periodically repeated elementary cells. The transport equations are derived for momentum and mass which are averaged in the elementary cells. Thus the velocity field does not have to be resolved locally. A calculation example is presented for a ceramic monolith and compared to measurements with capacitance tomography. The presented model is uselful for fluid dynamic scaleup calculations for reactors. The application of the model can easily be extended to countercurrent flow in structured packings.