Extended trading is undertaken on electronic market exchanges either before or after regular trading hours.

In the United States, pre-market trading occurs between 8:00 a.m. and 9:30 a.m. Eastern Time (ET), and after-market trading typically occurs between 4:00 p.m. and 6:30 p.m. ET. After-hours trading is usually abbreviated on message boards with the acronym AH.

Until recently, extended trading volume was relatively low. However, the volume of extended trading has exploded, as the Internet and related computer technologies have become more prevalent and the markets have grown more international.

Once the preserve of big institutional investors who had the confidence to deploy unorthodox methods, extended trading is increasingly adopted by retail investors, who are becoming more comfortable with the interconnected electronic communications networks that make it possible to trade at unconventional hours. Online trading is less of a novelty and more commonplace, removing the mystique of extended trading.

Extended hours trading allows nimble investors to act quickly to major events that are investment "catalysts," such as sudden corporate misfortune, political turmoil overseas, terrorist attacks, etc.

Trading before or after the markets officially open allows instantaneous investment decisions that are pegged to the latest developments. A caveat is that extended hours trading can be subject to the emotional whims and fears of less-seasoned, smaller investors—consequently, veteran pros on Wall Street sometimes derisively refer to extended trading as "amateur hour."