The contributions to this special volume were selected in order to show present trends in the field of bioprocess monitoring and estimation. Established conventional methods are critically discussed and non-conventional ones introduced. Two papers focus on more fuzzy approaches such as electronic noses or advanced chemometric techniques. One contribution illustrates the high potential with the example of cephalosporin production. Three contributions dare to "look" inside the cells, one by the analysis of (microscopic) images, one by trying to estimate the so-called physiological state, and the third by analyzing the metabolic network. This shows how sophisticated (bio)chemical as well as mathematical analytical tools result in better understanding of living systems and bioprocesses.