Advances in the field of cell biology have always been closely related to the development of quantitative analytical methods that can be applied to individual cells or cell organelles. Almost from the early stages following the invention of the microscope, the investigator has been keenly interested in obtaining informa­ tion on the functionality of single cells and how cells perform under different sets of experimental conditions. Although cells could be viewed in the microscope for a few hundred years, only since the relatively recent application of autoradiography did we come to realize that, although cells may visually appear very much alike, they are quite different in their functional capacity. The quest to understand these differences in a cell population lead to a new series of techniques for labeling and quantitating DNA content and similar approaches have driven the develop­ ment of methods for analyzing various other cellular properties. The development of new analytical techniques follows the age old pattern of applying successes of the past with current inno­ vation, logic and new biological information. Results from auto­ radiography expanded the concept of the cell cycle from inter­ phase and mitosis to the more definitive GO/GI, Sand G2/M phases. This new knowledge lead to the development of techno­ logy to measure and analyze various parameters related to the cell cycle.