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and during television infomercials, companies have been hyping the supposed big returns of the global currency game, nicknamed forex. Even some New York City subway cars carry ads championing this marketplace.

Individual investors can be forgiven for not knowing much about forex short for foreign exchange and they probably will never need to. Exploiting the subtle fluctuations between the world's monies has traditionally been the realm of big institutions and hedge funds. Indeed, several trillion dollars in trades happens every day. Only sophisticated individuals with cash to burn dare place any bets. The rest could do just fine with other investments.

But for those who simply can't resist, the exchange-traded-fund industry has made this exotic part of the market accessible to Main Street. Over the last several years around a dozen currency ETFs launched that track the U.S. dollar, the euro, the Japanese yen and even the Swedish Krona. Now, anybody with a computer and a trading account can get in on what used to be reserved for the big boys. Around $4 billion sits in these funds despite most of them being around for just two years.

That money has flowed into these ETFs for one main reason: a hedge against the falling dollar. The U.S. dollar has been slowly decreasing in value for much of the last decade. The last year, though, that slide accelerated thanks, in part, to high energy prices, trade and budget deficits, the subprime and credit crises and the war in Iraq. It now takes $1.54 to purchase a single euro, a record low. If investors had anticipated that drop 12 months ago and purchased the CurrencyShares Euro Trust ETF, they'd be enjoying a 20% return vs. a 1% decrease for the S&P 500.

Despite that seemingly sweet gain, most investors can avoid currency ETFs entirely. Indeed, we don't recommend them for the average person even in small doses. Even though these ETFs can effectively diversify the typical portfolio, there are plenty of other ways to accomplish that goal and do it with less risk. Currency ETFs are complex and at times volatile. Even pros have a hard time getting their heads around this marketplace. "The currency market can have a slightly different correlation to the stock market and it can add some diversity," says Geoffrey VanderPal, a financial planner in Las Vegas who thinks the dollar could fall another 10% to 15% in 2008 and 2009. However, he adds, "Most of my clients don't fit the mold."

That hasn't stopped many advisors like VanderPal from checking them out. If you are, too, then you need to do some homework before you can make an astute judgment. Advisors and individuals must look outside the usual criteria they use to evaluate stocks or mutual funds. Indeed, the direction of a given currency will depend in large part on a country's economic growth, its trade balance, its inflation rate and its current political leadership. So much for relying on earnings-per-share projections or price/earnings ratios.

Rydex, one of the biggest players in this space, has eight CurrencyShares ETFs. Its Euro Trust and Japanese Yen funds contain $1 billion in assets each. "We knew currencies were an asset class that was being underserved," says Ed Lopez, director of ETF strategies at Rydex. (Barclays'iPath

CurrencyShares ETFs own a corresponding amount of a given underlying currency. For example, a share in the CurrencyShares Euro ETF is the equivalent of 100 euros. These funds are designed to make money in two ways. First, they kick off an interest rate. CurrencyShares Australian Dollar ETF, for example, yields 6.5%, while the Japanese Yen product offers a modest 0.23%. U.S. investors can also reap rewards when a given currency increases in value against the dollar. When the position is sold, the appreciated currencies eventually translate into more dollars in your account. Of course, if the dollar gains value against a given currency it becomes more expensive to purchase that underlying money.

And therein lies a lot of the risk. It's tough to predict which way currencies will go. And even if you guess right it can take years for the scenario to play out. Indeed, the dollar has been falling for much of the 2000s. It was only this year that it really dropped. "You can be right about the direction and still be wrong for a couple of years," says Ray Benton, a financial advisor in Denver.

To minimize some of that impact some of it investors could opt for a broader basket of currencies instead of betting on just one. The PowerShares DB G10 Currency Harvest ETF holds a portfolio of futures contracts on currencies like the dollar, the euro, the yen, the Swiss franc and the British pound. The ETF will then short the three currencies whose countries have the lowest interest rates while going long the three currencies from countries with high interest rates. The theory is simple: Currencies associated with higher interest rates tend to increase in value compared to those associated with lower rates. This fund has returned a decent 7% the last year while charging $75 for every $10,000 invested.

DB US Dollar Index Bullish) and "bearish" (DB US Dollar Index Bearish) ETFs that go long and short the dollar, respectively. ProShares has filed with the SEC to launch a series of "ultra" ETFs that will try to double the returns of a given currency or try to ring up the inverse of those results.