Cell culture monitoring is essential for the continuous characterisation of cultivated cells. This helps us to understand each cell line's growth and metabolism properties and to establish and ensure reproducible cell cultivations. Monitoring not only comprises biological parameters, like cell count and viability, but also physiological parameters, like substrate, metabolite and possibly product concentrations. Furthermore, a large set of physicochemical parameters such as pH, temperature and osmolarity is routinely monitored in cell culture applications. A multiplicity of monitoring technologies exists. Often, there are several methods for one monitoring parameter to choose from and the choice depends on application and process needs. Moreover, new methods are developed at ever increasing speed. Today, cell culture processes and regulatory agencies demand detailed process understanding, which extensive monitoring is a prerequisite for. In this chapter, the current state of monitoring technologies and applications is reviewed. Particular emphasis is placed on biological parameters, i.e. cell density and viability as well as substrates, metabolites, and product concentration but also on cell stress and apoptosis. Furthermore, promising exploratory technologies are surveyed. Lastly, the chapter is meant to bridge the gap between existing technology-driven and more biologyoriented publications in this field.