Motivated by the behavior of internet stock prices in 1998-2000, we present a continuous time equilibrium model of bubbles where overconfidence generates disagreements among agents regarding asset fundamentals. With shortsale constraints, an asset owner has an option to sell the asset to other agents who have more optimistic beliefs. This re-sale option has a recursive structure, that is, a buyer of the asset gets the option to resell it. This causes a significant bubble component in asset prices even when small dierences of beliefs are sucient to generate a trade. The model generates prices that are above fundamentals, excessive trading, excess volatility, and predictable returns. However, our analysis shows that while Tobin’s tax can substantially reduce speculative trading when transaction costs are small, it has only a limited impact on the size of the bubble or on price volatility. We give an example where the price of a subsidiary is larger than its parent firm. Finally, we show how overconfidence can justify the use of corporate strategies that would not be rewarding in a “rational” environment.