A mere 15 years ago, selecting the right exchange-traded fund (ETF) was no big challenge. That's because the first ETF wasn't introduced until 1993, and the second didn't follow until 1995. Since then, however, the growth rate among these versatile investment vehicles has been exponential - so fast, in fact, that the monitoring firm Morningstar now tracks the performance of 854 ETFs, with new funds being added almost weekly. So, from this mushrooming roster of new ETFs - now covering virtually every market sector, both domestic and international - how do you select the right one (or, more likely, ones) for your portfolio? If you're not already familiar with ETFs , here are four reasons why you should consider adding some balance to your portfolio.