Square planar platinum(II) polypyridyl complexes have demonstrated promise in a wide range of applications due to their interesting ground and excited state properties. It is therefore vital to understand how to control the physical and chemical properties of such metal-organic chromophotes in order to be able to tune their photophysical properties according to application-specific requirements. This book focuses on the photophysical, electrochemical, and spectroelectrochemical properties of a variety of organometallic platinum(II) polypyridyl complexes which possess accessible ligand-centered and charge transfer excited states as well as the basic information necessary to understand these properties. In addition to the interesting solution phase properties, metal-metal interactions in the solid state can be profoundly perturbed by vapor adsorption, rendering significant changes in color and photoluminescence of the compounds. Vapochromic sensing applications based on platinum(II) terpyridyl compounds will also be presented in this book.