This paper examines the extent of firm level over-investment of free cash flow. Using an accounting-based framework to measure overinvestment and free cash flow, I find evidence that, consistent with agency cost explanations, over-investment is concentrated in firms with the highest levels of free cash flow. Further tests examine whether firms' governance structures are associated with over-investment of free cash flow. The evidence suggests that certain governance structures, such as the presence of activist shareholders, appear to mitigate over-investment.Keywords Free cash flow AE Over-investment AE Agency costs JEL Classification G3 AE M4 This paper examines firm investing decisions in the presence of free cash flow. In theory, firm level investment should not be related to internally generated cash flows (Modigliani & Miller, 1958). However, prior research has documented a positive relation between investment expenditure and cash flow (e.g., Hubbard, 1998). There are two interpretations for this positive relation. First, the positive relation is a manifestation of an agency problem, where managers in firms with free cash flow engage in wasteful expenditure (e.g., Jensen 1986;Stulz 1990). When managers' objectives differ from those of shareholders, the presence of internally generated cash flow in excess of that required to maintain existing assets in place and finance new positive NPV projects creates the potential for those funds to be squandered. Second, the positive relation reflects capital market imperfections, where costly external