The recent observed tropical expansion has similar magnitudes in the annual mean in the Northern and Southern Hemispheres, but models suggest that the factors driving the expansion differ between the hemispheres. In the Southern Hemisphere, increasing greenhouse gases and stratospheric ozone depletion contributed to tropical expansion over the late 20th century, and if greenhouse gases continue increasing, the Southern Hemisphere tropical edge is projected to shift further poleward over the 21st century, even as stratospheric ozone concentrations recover. In the Northern Hemisphere, the contribution of increasing greenhouse gases to tropical expansion is much smaller and will remain difficult to detect in a background of large natural variability, even by the end of the 21st century. To explain comparable recent tropical expansion rates in the two hemispheres, natural variability must be taken into account. Recent coupled atmosphere-ocean variability, including the Pacific Decadal Oscillation, has contributed to tropical expansion in both hemispheres, but particularly in the Northern Hemisphere. However, even if observed sea surface temperatures are prescribed, modeled tropical expansion rates still vary widely due to internal atmospheric variability.