Chemiluminescence is a fascinating and mesmerizing phenomenon, which consists on the generation of light as the result of a chemical reaction. This process has been gaining attention in the last decades, being now a standard tool in analytical, bioimaging, biomedical and pharmaceutical applications. This book provides a detailed overview of the basic mechanisms and principles of the most relevant chemiluminescent systems, as well as describing the most recent advances and applications. Written by experts, this is a reference text suitable both for researchers focused on chemiluminescence and for those wanting to start working on this field. In Chapter One, the progress and evolution of the chemiluminescent studies in imidazopyrazinone-based compounds are reviewed, with emphasis on their potential as diagnosis tools. Chapter Two addresses the spectral changes associated with firefly bioluminescence by focusing on the tautomeric equilibria of the light emitter. Chapter Three reviews, for the first time, the liquid-phase chemiluminescence of fullerene and derivatives. Chapter Four is devoted to studies of chemiluminescence with the participation of lanthanide ions. Chapter Five summarizes the mechanism behind the chemiluminescence of acridinium esters, while discussing their current applications. Chapter Six explains the potential of phenothiazine derivatives as enhancers in analytic chemiluminescent assays. Chapter Seven collects the available information of both well-known and lesser-known examples of squid bioluminescence. Chapter Eight discusses the enhancement of chemiluminescent emission by using metal-nanoparticles near a luminescent molecule, which is known as metal-enhanced chemiluminescence. In Chapter Nine it is explained how model molecules can make the study of complex bioluminescence reaction mechanisms accessible, as well as allowing for chemiluminescence on macroscopic molecular crystals. Chapter Ten describes experimental and computational approaches used for enhancing the thermostability of firefly luciferase. Chapter Eleven demonstrates the usefulness of theoretical studies for better understanding chemiluminescent reactions. Chapter Twelve describes the fundamental theoretical aspects of state-of-the-art multiconfigurational methods and shows illustrative examples of their application to the study of chemiluminescence. Finally, Chapter Thirteen discusses chemiluminescent experiments that are simple, inexpensive and can be performed by students.