It is generally understood that the pattern of repeated expiration and short-term renewal of the federal production tax credit (PTC) causes a boom-bust cycle in wind power plant investment in the US This on-off pattern is detrimental to the wind industry, since ramp-up and ramp-down costs are high, and players are deterred from making long-term investments It is often assumed that the severe downturn in investment during "off" years implies that wind power is unviable without the PTC This assumption turns out to be unsubstantiated: this paper demonstrates that it is not the absence of the PTC that causes the investment downturn during "off" years. but rather the uncertainty over its return. Specifically, it is the dynamic of power purchase agreement (PPA) negotiations in the face of PTC renewal uncertainty that drives Investment volatility. With contract negotiations prevalent in the renewable energy industry, this finding suggests that reducing uncertainty is a crucial component of effective renewable energy policy The PTC as currently structured is not the only means, existing or potential, for encouraging wind power investment Using data from a survey of energy professionals, various policy instruments are compared in terms of their perceived stability for supporting long-term investment (C) 2010 Elsevier Ltd All rights reserved.