The rapid developments in image cytophotometry in the late 1950s and early 1960s and the in flow cytometry a decade later roused great expectations among researchers in biological and medical sciences. Image cytophotometry made it possible for the first time to measure many different parameters both qualitatively and quantitatively in combination with morphological studies. A second major step forward occurred when flow cytometers, originally developed in a few research laboratories, became commercially available in the early 1970s, permitting automated cellular analysis and sorting at extremely high speed. Although the flow systems, sources of excitation light, and methods for detecting emitted light are basically the same today as 20 years ago, the almost exponential development in certain related fields of research has led to entirely new opportunities for this technology. Through modern computer technology, the tasks of data acquisition, handling, and analysis have been enhanced enormously. In particular, multiparameter analysis during cell measurement and sorting, as well as the ability to analyze listmode data, have given new dimensions to the field. The number and scope of immunological and immunohistochemical techniques utilizing monoclonal and other antibodies have greatly expanded, and at the same time these techniques have become widely available as everyday tools for any cytometric researcher. Molecular biology and genetics have likewise added another dimension for applications of cytometric methodology. Perhaps the greatest impact has been in the application of flow cytometry to cell biology. Improvements in cell kinetic measurements—e.g., using BrdU and analysis of cellular DNA—combined with other techniques for identifying subpopulations of cells have helped make cytometry an important tool for anyone involved in studies of cell growth. At the other end of the cellular life cycle, automated measurements of apoptosis have given insight into the complex equilibrium between cell proliferation and cell loss in any tissue. In parallel with the development of all these new, directly applicable methods, flow cytometers have become more reliable and user friendly, opening up this powerful methodology to anyone who studies eukaryotic and prokaryotic cells. One important result has been that clinical applications of cytometry as an adjunct to pathology, cytology, and hematology are expanding rapidly. As the breadth of both the technology and applications of cytometry expanded, however, the need for standardized methods for image and flow cytometry became increasingly crucial. Experimenters newly drawn to cytometry needed guidance in applying it to their particular research areas; moreover, the cytometric literature has expanded so rapidly that even for those actively working in the field, keeping track of recent developments has become a nearly unmanageable task. In response to this problem, in 1990 the International Society for Analytical Cytology (ISAC) first took the initiative to assemble the original Handbook of Flow Cytometry Methods. The continuing pace of change has necessitated several revisions. The latest descendent of that handbook is the present more complete and updatable volume, Current Protocols in Cytometry, which presents an abundance of advanced methodology that will be of great utility to a wide circle of readers. The combination of introductory chapters and general overviews on the one hand and specific methods on the other makes this a valuable resource for newcomers to the field and experienced researchers alike. Perhaps most importantly, the manual is designed to evolve over time along with the methods it describes: its looseleaf format and quarterly updates allow new techniques in emerging areas to be added and existing material to be revised as procedures are improved and expanded. Moreover, this volume as well as the updates are also available on CD-ROM, a format that permits readers to conveniently access the numerous cross-references within the manual by hypertext links, perform context-based searching, and make individualized notebooks containing frequently used protocols. More than ever, education and training are vitally important tasks in modern scientific disciplines. In addition to fulfilling these goals, this volume also presents a forum for standardization and exchange of methodology in an area of research that is essentially interdisciplinary. Not least, the continuous updating and expansion of Current Protocols in Cytometry will make it an important tool in the forefront of this type of science now and in the future.