The entry of chlorine into the mainstream of atmospheric photochemistry was in progress in the early 1970s when the Rowland-Molina hypothesis linked the release of fluorocarbons at the surface to the gas phase catalytic destruction of ozone in the stratosphere. This chapter provides an analysis of the interplay between chemical kinetics and physical dynamics. It emphasizes prognostic elements of the subject by comparing observations with model calculations. The chapter also emphasizes model independent diagnostic studies of the real atmosphere which have the fundamental advantage of testing basic hypothesis of chemical cause and effect through direct observation and can thus define the accuracy of perturbations to the system. Many aspects of the model independent approach can be effectively applied only to the chlorine-ozone system. The chapter discusses the essential elements of chlorine chemistry in parallel with physical transport processes which, while inseparably related to chemical changes, need not obscure or dilute crucial conclusions obtained from observations.