Nitrification has been studied extensively for its significance in the nitrogen cycle and within the treatment of wastewater. Often nitrogen removal from wastewater is problematic because of the low growth rare of the bacteria involved. Immobilization is an efficient method to retain slow-growing organisms in continuous-flow reactors. Immobilized cells can be divided into "naturally" attached cells (biofilms) and "artificially" immobilized cells. Biofilm systems are widely used in practice. Immobilized-cell reactor cells entrapped in gel beads form an alternative to these attached-cell systems. An overview is given of studies with immobilized nitrifying cells, with special attention to model development and evaluation. Dynamic models, including growth, diffusion limitation in the support material and in the microcolonies, and external mass transfer, are discussed. Experimental evaluation of the models is essential for their control if the models provide insight in the complex processes and for their use as a tool in reactor design. Several techniques for evaluation are given, including macroscopic substrate consumption rates and local analyses in the support material.