The appearance of ferrocene in the middle of the 20th century has revolutionized organometallic chemistry and is now providing applications in areas as varied and sometimes initially unexpected as optical and redox devices, battery and other materials, sensing, catalysis, including asymmetric and enantioselective catalysis, and medicine. The author presents here a general, although personal, view of ferrocene's chemistry, properties, functions, and applications through a literature survey involving both historical and upâtoâdate trends and including examples of his group's research in a number of these areas. The review gathers together general features of ferrocene chemistry and representative examples of the salient aspects. Its focus is on ferrocene's basic properties, ferroceneâcontaining ligands, the ferrocene/ferricinium redox couple, ferrocene mixedâvalence and averageâvalence systems, the ferricinium/ferrocene redox shuttle in catalysis, ligandâexchange reactions, ferroceneâcontaining polymers, ferroceneâcontaining structures for cathodic battery and other materials, ferrocenes in supramolecular ensembles, liquid crystals, and nonlinear optical materials, ferroceneâcontaining stars and their electrostatic effects, ferroceneâcontaining dendrons, dendrimers, and nanoparticles (NPs) and their application in redox sensing and catalysis, and ferrocenes in nanomedicine.