This chapter provides a general introduction for the application of mechanistic and statistical models in bioreactor process development and optimisation. We first introduce the equations governing the behaviour of batch and continuous stirred tank reactors (CSTR) for a given chemical system. After presenting the criteria to generalise these concepts to biological systems and bioprocesses, we discuss the implementation of mechanistic models for the simulation and control of the bioreactor performance with particular emphasis on product quality attributes, such as N-linked glycosylation. In addition, we describe the implementation of statistical and hybrid models and their application in process development and reactor optimisation. Lastly, we compare the use of the various modelling techniques for process monitoring and control.