Total assets invested in exchange-traded funds listed in the United States rose 4.2% over the past month, according to the Investment Company Institute. (ETFs are relatively new securities that trade like stocks and are usually tied to an index of some sort, such as the S&P 500 index.) The current total invested amount? $335 billion. On the off-chance that that doesn't seem like a lot, consider that there are about 6.5 billion people on earth, so that would average out to about $50 per person. Of course, I hope you realize that about half the world lives on less than $2 per day (and that you send some aid their way now and then -- perhaps through our annual Foolanthropy campaign). So it's safe to assume that half the world isn't invested in U.S.-based ETFs. And truth be told, it's not just those who live on less than $2 a day who aren't invested in ETFs. Still, the number of ETF investors is growing.

That $335 billion sum is more than the market capitalization of Microsoft or Wal-Mart, and it's growing faster than the values of these companies lately. (ETF assets in total are up some 50% over the past year or so.) Within the ETF universe, internationally focused ETFs drew more money, rising $7.4 billion to $88.3 billion. Domestic ETFs rose $5.5 billion to $230.4 billion.

There are now more than 200 ETFs based in the U.S., and the number is growing. I've discussed some new ETF developments in these articles:

So, is there any reason for you to jump on this charging bandwagon and snap up some ETFs for yourself? Well, ETFs do offer some advantages. Whereas some index funds sport minimum investment amounts of $1,000 or $3,000, or more, you can buy as little as a single share of an ETF, which might go for $150 or $100 or less. Also, they can give you instant, diversified exposure to lots of regions and industries. One of the best-performing ETFs of 2005, for example, was the Energy Select Sector SPDR (AMEX:XLE), which popped up 40%. Its top holdings include ExxonMobil (NYSE:XOM), Chevron, ConocoPhillips (NYSE:COP), Schlumberger (NYSE:SLB), Halliburton, Marathon Oil (NYSE:MRO), and Valero Energy (NYSE:VLO). If you want to be instantly invested in, say, a bunch of dividend-paying stocks, or emerging-market stocks, or healthcare stocks, there are ETFs that can help you do so.

Before you jump, though, take some time to learn more about whether ETFs are really for you. Our ETF Center will teach you a lot. It features info on how ETFs stack up against mutual funds, how to develop an investment strategy with ETFs, pitfalls to avoid, and how to avoid ETF imposters.

These articles may also be of interest:

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Longtime Fool contributor Selena Maranjian owns shares of Wal-Mart and Microsoft. The Motley Fool has an ironclad disclosure policy .