If you have read the whole of this book I salute you. If you are not primarily involved in this field, have read the whole book and have understood everything, then I salute both of us. The intention was an introduction and, indeed, that is all that it is as, on the applications front, I have omitted very considerably more than I've included. The problem with writing about flow cytometry is that it covers the whole of biological science and it is not possible to include everything, thus a considerable degree of selection must be employed. However, I hope that the first 10 chapters, which were intended to show how these instruments work, how they can be used and some of the associated problems, were not only intelligible but also prove to be useful. The last five chapters were intended to show where they can be used in a number of fields in cell biology including oncology. However, the biggest applications omission is the whole of immunology which, if it had been included, would have more than doubled the size of the book. The technology is also being used increasingly in microbiology, plant biology and in the aquatic sciences and none of these have even been mentioned up till now. The last of these is assuming increasing importance and in recognition of this the September 1989 issue of Cytometry, containing 21 papers and over 600 references, was devoted to this topic.