Regulators around the world are concerned that growth in dark trading may harm price discovery. We show that block and non-block dark trades affect price discovery differently. We find no evidence that block trades in the dark impede price discovery. In contrast, high levels of non-block dark trading harms price discovery and reduces the informational efficiency of prices, while low levels of non-block dark trading can be beneficial. One reason non-block dark trading can be harmful is that the lack of pre-trade information reduces the market’s ability to infer and incorporate private information. Uninformed trades are more likely to execute in the dark, which increases adverse selection risk and bid-ask spreads in the transparent exchange.