Federal courts are currently split, even within particular districts, on the basic question of what a plaintiff must show to establish that a defendant’s conduct constitutes trademark dilution by blurring. Federal trademark law defines “dilution by blurring” as “association arising from the similarity between a mark or trade name and a famous mark that impairs the distinctiveness of the famous mark.” In construing this statutory language, a majority of courts have held that, to establish blurring, a plaintiff need only show that consumers associatethe defendant’s mark with the plaintiff’s famous mark. These courts appear to assume that, to the extent that there is consumer association, this association alone will “impair[ ] the distinctiveness” of the famous mark. A minority of courts have held that the plaintiff must show bothconsumer association andthat the consumer association “impairs the distinctiveness” of the famous mark. In this Article, we make three contributions to the current debate over what must be shown to establish dilutionby blurring. First, we report the results of a set of experiments that reveal that the majority approach is fundamentally deficient. These experiments demonstrate that, even when consumers associate a junior mark with a famous senior mark, this association does not necessarily result in any impairment of the ability of the senior mark to identify its source and associations. Second, we set forth a new method for determining when association is likely to lead to impairment. This method, which we term the “association strength test,” evaluates changes in how strongly survey respondents associate a mark with its source or attributes upon exposure to a diluting stimulus. Third, we evaluate the current state of the art in trademark dilution survey methodology: response time surveys. These surveys purportedly show both consumer association and impairment. Through a set of experiments, we demonstrate that these surveys currently use the wrong control and are invalid. In light of our findings, we reflect more generally on the question of whether dilution by blurring ever occurs and on how the blurring cause of action may be reconfigured to better comport with courts’ intuitions about the true nature of the harm that the cause of action seeks to address.