Several recent studies suggest that the Tropics have been expanding over the past few decades and that this widening may continue into the future in association with anthropogenic climate change. We will summarize our observationally-based findings of an increase in the width of the tropical belt of several degrees of latitude since 1979. These results are based on distinctions between the tropics and extratropics in total-column ozone and tropopause height, location of the subtropical jet streams, and identification of the poleward edges of the Hadley circulation in outgoing long-wave radiation and mean meridional mass streamfunction. We will also summarize climate model projections for the 21st century showing tropical belt expansion. These suggest that anthropogenic forcings may be expected to lead to an expansion of the Tropics by 2 degrees latitude or less. Thus the observed changes during the late 20th century are already as large as, or larger than, the projected changes during the 21st, raising questions about the causes of and processes associated with the observed and modelled changes. We will discuss the scientific and societal implications of an expanding tropical belt and outline research directions to further our understanding of this phenomenon.