Various measures of tropical cyclone activity exhibit substantial interdecadal variability in the North Atlantic and western North Pacific, where they have been best observed. Particularly in the Atlantic, there is a high correlation between this variability and changes in the late summer and early fall tropical sea surface temperature. In this talk, I will discusss the various environmental factors that appear to influence interdecadal variability of tropical cyclones, and show that in the Atlantic there is substantil cross correlation among these factors, yielding an apparent sensitivity to sea surface temeprature greatly in excess of theoretical calculations of its direct effect. I will then review evidence for natural versus anthropogenic causes of this variability.