Soaring prices in European alternative energy stocks and their subsequent tumble have attracted attention from both investors and academics. This paper by Martin T. Bohl, Philipp Kaufmann and Pierre L. Siklos extends recent research to an international setting and analyzes whether the explosive price behaviour of the mid-2000s was driven by rising crude oil prices and an overall bullish market sentiment. Inflation-adjusted US alternative energy stock prices do not exhibit signs of explosiveness. The authors find strong evidence of explosive price behaviour for European and global sector indices, even after controlling for a set of explanatory variables, while the sector indices plunged with the outbreak of the global financial crisis, idiosyncratic components continued to rise and did not start to decline until after world equity markets had already begun to recover in 2009. This finding suggests a substantial revaluation of alternative energy stock prices in light of intensifying sector competition and shrinking sales margins, and casts some doubts on the existence of a speculative bubble. The authors observe temporary episodes of explosiveness between 2005 and 2007 followed by rapid collapses, indicating the presence of some irrational exuberance among investors.