In Canada, tax incentives have been recently introduced to promote physical activity and reduce rates of obesity. The most prominent of these is the federal government's Children's Fitness Tax Credit, which came into effect in 2007. We critically assess the potential benefits and limitations of using tax measures to promote physical activity. Careful design could make these measures more effective, but any tax-based measures have inherent limitations, and the costs of such programs are substantial. Therefore, it is important to consider whether public funds are better spent on other strategies that could instead provide direct public funding to address environmental and systemic factors. (Am J Public Health. Published online ahead of print June 16, 2011:e1-e7. doi:10.2105/AJPH.2011.300201).