Complex processes routinely consist of several individual sub-processes interacting in tandem. Numerous examples are found in biology and technology, namely the coupling of metabolic and regulatory networks in cellular systems, or the combination of processes or individual process units in biochemical engineering.

Analysis, design and optimization of coupled processes require not only a detailed understanding of the structural and dynamic properties of the individual subprocesses but also a thorough characterization of the interaction of all subunits. Complexity further increases when behavior of the coupled overall process can not be correctly predicted through dependence upon previous knowledge of the individual units, particularly when a qualitatively new behavior emerges from the coupling. As an illustrative example, consider two stable subsystems, which, after coupling, result in one unstable overall system. It is to be emphasized that the abstract methods of systems theory, which are essentially and originally independent of any particular application, are of crucial importance when coupled processes are under consideration.

As far as chemical engineering systems are concerned, not only overall yields and product purity but also material and energy recycles between individual process units account for an optimal utilization of raw materials and energy alike. Typical examples are reactor-separator-systems where unconverted reactants are separated from the products and subsequently recycled to the reactor or integrated concepts for the downstream processing of pharmaceuticals. The common interest in all these cases is directed towards optimal design and control of the respective coupled processes.

In addition, in biotechnological applications, prokaryotic and eukaryotic cells are of considerable interest. Bacteria, fungi, yeast, and mammalian cells are extensively used for the production of a wide scale of products ranging from simple organic compounds to highly specific pharmaceuticals.

A detailed analysis of factors influencing cell growth and product formation contributes to optimization of bioprocesses. In addition, targeted manipulations of the genome of production cell lines would enhance specific productivity and, in conjuncture with suitable process control strategies assist the increase of biotechnological process yields. In addition, progress in understanding biological systems on a cellular level is prerequisite for further advances in medicine. The detailed analysis of the complex metabolic, regulatory and signal transduction pathways of mammalian cells, for instance, not only elucidates the properties of cellular networks, but also assists in the identification of possible targets for drug development, contributing to an enhanced understanding of the underlying cause of many diseases.