After-hours trading is the activities of electronic trading that take place after the close of the formal trading session. Trading after these formal hours was previously an extravagance, accessible only by high net-worth investors and institutional investors. But that changed by the turn of the new century. It was the introduction of electronic communication networks (ECNs) that has enabled everyday individual investors to participate in after-hours trading. Sometimes, after-hours trading refers to trading a listed security over the counter (i.e. not through organized exchanges) after the regular trading hours of the main exchanges.

Though traders can take advantage of any developments that break out after official trading hours, without having to wait for next day opening, AHT is associated with certain serious cautions. Trading after closure of the market brings about fewer number of market participants than when the exchanges are open. Therefore, the market becomes less liquid, leading to wider bid-ask spreads. In the meantime, prices show wider fluctuations, and market volatility picks up.