Interest in atmospheric gaseous chlorine (GCl) has been generated by its increasing anthropogenic introduction coupled with its reactivity with ozone. We describe present and anticipated future sources of GCl and derive an estimate of their stratospheric source strength. The stratospheric sources are transport of tropespheric GCl across the tropopause, direct stratospheric introduction by major volcanic eruptions, and anthropogenic emissions in the stratosphere. The mean source strengths are estimated to be, respectively, 4 × 10¹⁰, 1 × 10¹¹, and 5 × 10⁹ g GCl yr⁻¹, the last figure representing a projection of anthropogenic emissions to include 50 space shuttle flights per year. Whereas the first source introduces GCl only at the base of the stratosphere, the other two can supply GCl directly to altitudes up to ≈50 km. The dominant, volcanic, source is highly variable, it being estimated that as much as 3 × 10¹³ g GCl can enter the stratosphere from a single eruption. Although major volcanic eruptions are indicated to be the dominant source, few ‘hard’ data are currently available.