•Structured reactors allow high precision at all relevant length scales.•Monoliths are the state-of-the art reactors in the majority of environmental catalysis processes.•In multiphase applications monoliths are ready for extensive practical use, in particular in the segmented flow regime.•Monolith stirrer reactors are suitable for small-scale productions in the liquid phase.•Single capillary reactors are convenient, flexible and cost-effective.Structured reactors offer high precision in catalysis at all relevant length scales of the catalytic process, from the catalytic species up to the reactor. They offer unusual freedom in design with respect to diffusion length, hydrodynamic regime and reactor configuration. Monoliths are the prime example of such systems.