We use observed insider trades to assess the economic determinants that explain the enforcement of company-imposed quarterly open trading windows. We find that insider trading restrictions reflect concerns about information asymmetry, the strength of external monitoring, and executives’ liquidity needs. We also show that enforced trading windows constrain opportunistic insider trading activity, with insiders generating larger trading profits when boards set trading policies that are abnormally loose. We also identify and explore the enforcement of event-specific “ad hoc blackout windows,” by firms engaged in material corporate events. Interestingly, the absence of trading in these windows is associated with contemporaneously higher information asymmetry. These periods are then followed by increased trading volume and higher stock returns, suggesting investors may not immediately incorporate all information conveyed by unscheduled trading restrictions.