The first edition of Principles was published in 1983, and adigm of fluorescence, from a reliance on organic fl- the second edition 16 years later in 1999. At that time I rophores, to the use of genetic engineering, nanotechno- thought the third edition would not be written until 2010 or gy, and near-field optics. later. However, the technology of fluorescence has I wish to express my appreciation and special thanks to advanced at an accelerating pace. Single-molecule detec- the individuals who have assisted me in the preparation of tion and fluorescence-correlation spectroscopy are becom- the book. These include Ignacy Gryczynski for assistance ing almost routine. New classes of probes have appeared, with the figures, Krystyna Gryczynski for drawing the f- such as the semiconductor nanoparticles, or QDots, and ures, Joanna Malicka for proofreading the chapters, Kazik genetically engineered green fluorescent probes. Addition- Nowaczyk for the cover design and color digitizing of all ally, it is now becoming possible to control the excited figures, Tim Oliver for typesetting, and the NIH for their states of fluorophores, rather than relying only on sponta- support of my laboratory. And finally, Mary, for her endless neous emission. These developments are changing the par- hours of typing, correspondence and support. Joseph R.