The performance characteristics of moving-bed biofilm reactors (MBBRs) have been analyzed both mathematically and experimentally. Both two phase operation (lactic acid synthesis from cheese-whey permeate) and three phase operation (Xanthan gum production) in both batch and continuous flow reactors have been studied. Mathematical simulation was performed considering the heterogeneous nature of the system with appropriately defined effectiveness factor being incorporated to account for resistance to substrate transfer into biofilm. The flow reactors were modeled based on the tanks-in-series approach. The mathematical models (software packages) developed were adequately verified by comparing with experimental data. The interesting performance features of these reactors have been highlighted and the dependence of reactor performance on key system/operating parameters such as batch time/space time, catalyst loading and catalyst size has been well-illustrated. The limitation that these bioreactors are best suited mainly for small capacity installations has also been indicated.