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By the time you read this introduction to after-hours trading, it may be outdated.

That's because after-hours trading -- meaning the period outside the regular trading session, which runs weekdays from 9:30 a.m. Eastern time to 4:00 p.m. -- is rapidly evolving as the market tries to keep pace with technological advances and the demands of investors. Technology has come a long way from the days when the major markets closed every Wednesday so the exchanges could tally their transactions, and it is now helping push the market toward potentially a 24-hour, nonstop trading day.

The major stock markets, such as the

New York Stock Exchange

and the

Nasdaq Stock Market

, have mulled over extending their regular trading hours. But until they do, there are a host of electronics trading networks that will let you continue to trade until as late as 8 p.m..

After-hours trading carries vital differences from regular trading that investors need to be aware of -- namely, concerns about volatility and getting the best price. Because volume, or the amount of shares changing hands, is typically lighter after hours, a stock's price swings may be far more pronounced. Also, recent studies from academics and regulators have indicated that the average spread -- the difference in price between offers to buy and sell -- more than triples in after-hours trading, meaning investors may not be getting the best deal in buying or selling stock.

Even if investors don't trade after hours, it's useful to monitor trading outside the regular session. Companies often release earnings reports and other pivotal announcements after the 4 p.m. close; the after-hours activity in these companies' stocks often reveals whether or not Wall Street likes the news.

The Live-Long Day

Every investor, from the newbie to the pro, stands to benefit from keeping an eye on the after-hours market. For more on after-hours trading, check out the following:

An Extended-Hours Trading Primer Point-Counterpoint