The Production Tax Credit (PTC) is a large subsidy for investment in wind energy. While in place continuously since 1992, it was typically extended for a few years at a time.  The events leading up to each renewal created considerable policy uncertainty for the wind power industry, and we study the effect of this uncertainty on investment. The graph below shows the jump in wind capacity additions in the four years the credit was allowed to expire on Dec 31 (red lines) before being retroactively renewed. [back]