.For there to be progress in science, there must first be communication between experts of different disciplines. This is particularly true of modern biology which is becoming more and more of an interdisciplinary field. The present situation in cell biology clearly reflects this devel­ opment and demonstrates that the application of physical techniques was necessary before this field of biological research could be developed on an objective and quantitative basis. The utilization of optical phenomena as measuring parameters at the microscopic level has provided the basis for the development of quantitative cytochemistry. This rapidly growing extension of conventional cytochemistry and histochemistry is based on the visual oberservation of qualitative chemical criteria in correlation with the microscopically resolved structure of cells and tissues. Furthermore, the introduction into cytochemistry of such optical measuring techniques as ab­ sorption photometry, interferometry, and fluorometry, as well as the measurement of optical anisotropy, diffrac:tion and scattered light, has provided the methodological bridge for the ex­ change of knowledge between cell biology on the one hand and biochemistry, or molecular biology, on the other.