Book excerpt: An Introduction to Fluorescence Correlation Spectroscopy represents a comprehensive introduction to fluorescence correlation spectroscopy (FCS), a biophysical experimental technique increasingly used to study and quantify molecular mobility, concentrations and interactions in vitro, as well as in living cells and multicellular organisms. Students and researchers who are new to FCS can use the book as the first introduction to the technique, while those who are already using FCS regularly in their research may find it useful to deepen their understanding of the technique, its possibilities, limitations, and potential pitfalls as well as ways to avoid them. This book introduces the reader to all aspects of FCS needed for practical usage of the technique in their research. In the beginning the concept of fluorescence intensity fluctuations and their auto- and cross-correlation functions are explained to give readers an understanding of the underlying principles. This is followed by an overview of instrumental FCS setups and various ways of data collection and processing, the derivations of theoretical models relating the experimentally obtained correlation functions to the underlying molecular processes, and the description of the fitting of experimental data with those models. Mathematically more involved portions are separated from the rest of the text and can be easily skipped by readers more interested in the conceptual and practical aspects of FCS. The book contains interactive graphics and is accompanied by an interactive computable document file allowing the reader to test the dependence of FCS results on a variety of experimental parameters, and to gain practical insights into FCS data fitting. Key Features Introduces the concepts of FCS in an accessible way, supported by animations and graphics in the ebook. Includes a supplementary interactive computable document file that allows the reader to experiment with various FCS setup and fit parameters, allowing readers to test their understanding and simulate experimental outcomes. Provides rigorous mathematical derivations of fundamental FCS equations and models. Pedagogical features include questions, short reviews and critical discussions of literature relevant to the particular chapter that include applications and fundamental developments in the field of FCS.