Excited State Lifetime Measurements attempts to assist in clarifying and unifying the many characteristics and definitions of excited state lifetime measurements. The contents of this book are derived from a series of lectures presented to a research group in the University of New Mexico in 1967. The relevance as well as the methods and measurements of data treatment of excited state lifetimes are featured in this book. The first three chapters provide a brief discussion on concepts and applications of excited state lifetime measurements. Experimental methods and systems are also introduced in these chapters. Chapter 4 delves into more complex systems (serial decay kinetics and resonance energy transfer) while Chapter 5 focuses on the method of least squares fitting, its uses, and misuses. Chapters 6 to 8 mainly discuss the convolution integral and its different applications while Chapter 9 gives a more detailed presentation of instrumentation. The last two chapters discuss special errors and approaches to new methodologies regarding the study of the excited state lifetime measurements. The book will be useful to students and scientists including analytical chemists, photochemists, photobiologists, spectroscopists, and physicists.