There are many examples of applications of solid-state fermentation (SSF) that show promise in the laboratory, but relatively few examples of the development of successful commercial processes. This is partly due to the fact that SSF systems present various engineering challenges that have not yet been overcome. As a result, there are relatively few tools and guidelines for scale-up. This presentation will show that a better understanding is required of both “microscale” phenomena, namely the growth of the microorganism and the diffusion of gases, enzymes and nutrients within the substrate particle, and “macroscale” phenomena, namely bed properties and transport phenomena across the whole bed. With respect to microscale phenomena, we need a better understanding of how the growth of the microorganism is affected by the temporal variations in temperature, water activity and nutrient availability that are unavoidable in SSF processes. In agitated processes involving filamentous fungi, the effect of mechanical damage to the mycelium also needs to be better characterized. Only then will we be able to develop adequate kinetic equations. With respect to macroscale phenomena, we need a better understanding of how the growth of the microorganism leads to changes in the water sorption isotherm of the solids, the porosity of the bed and the properties of the air-solid interface. These changes affect the efficiency of transport phenomena and can lead to problems such as high pressure drops across the bed or channeling.