Several independent types of observations have revealed evidence of changes in the width of the tropical belt. Tropical widening, by several degrees of latitude over the past several decades, has been seen in radiosonde and reanalysis estimates of tropopause height, total column ozone observations, inferred cloud and convection patterns from out-going long-wave radiation observations, and atmospheric circulation patterns derived from reanalysis stream function data. Model simulations of past and future tropical belt width show both natural variability and secular changes forced by changes in greenhouse gases and stratospheric ozone. The potential connection between the observed changes and societally and ecologically important changes in global precipitation patterns has sparked interest in projecting future changes in the width of the tropical belt. However, discrepancies among the various observational estimates, incomplete characterization of their uncertainties, and the inability of models to completely explain the observations, make such projects very challenging. The seminar will provide an overview of these issues and identify outstanding questions.