Monolith reactors are attracting more and more attention as alternatives for both threephase slurry reactors [1-3] and trickle-bed reactors [4,5]. From a fluid mechanical point of view, the operating mode depends on the size of the straight parallel channels. In large channels the fluid trickles downwards along the channel walls and the gas travels through the channel in the channel core. In smaller channels, the dominant flow pattern is a segmented slug flow or bubble-train flow of alternating bubbles and slugs, where the bubbles span all but the complete channel cross-section. In the beginning of this chapter, criteria to predict the different multiphase flow regimes are briefly reviewed, and the remainder of the chapter deals with the segmented flow pattern. For the trickle-flow or film-flow pattern, the interested reader is referred to Chapter 13.