In this paper, the advantages and disadvantages of using segmented flow in microchannels to intensify biocatalytic processes are discussed. We consider both suspended cell cultures and immobilized enzymes. Once bubbles are formed in microchannels, they can no longer coalesce and, hence, no energy is required to break up larger bubbles. As a result, the same gas-liquid mass-transfer behavior can be obtained at an order of magnitude lower power input. The gas-liquid-solid mass transfer, i.e., to a catalyst on the channel walls, improves with decreasing energy input, allowing the combined reduction of power input and reactor size. These findings are supported by a scaling analysis based on the pressure drop work of Kreutzer et al.(1) and the mass-transfer work of Van Baten and Krishna.(2) For the practical application of monoliths for liquid-phase cell cultures, the stability of a downflow monolith reactor with respect to clogging by biofilm was investigated experimentally. The results indicated that, as long as flushing out of the suspended cells was prevented, stable operation was possible.