Quantification of microbial life is necessary for optimising bioprocesses. The scientific background of biologists, chemists and engineers, who are engaged in this field, is very specific and therefore the approaches to quantification are extremely different as well.

The roots for quantification and the constraints of microbial performance are discussed, and it is shown that only a holistic approach, the approach of systems biology, can lead to satisfactory results. The essential contribution of Wolfgang Babel to this holistic approach is emphasised.