When it comes to investing for your retirement, you might assume that you have two main choices: investing in individual stocks, some of which might grow very briskly, or investing in a broad-market index fund, which will deliver roughly the same return as the index it tracks. (Yes, you might also invest in actively managed mutual funds, but the bulk of them underperform their index counterparts over long periods, so let's ignore them for this article.) There's another alternative to consider, though: exchange-traded funds (ETFs), which are funds that can be traded like stocks and often track indexes. Here's a look at one ETF that might help grow your assets for retirement -- along with a related one that you might also consider. Image source: Getty Images. Meet the QQQ The Invesco QQQ Trust (NASDAQ: QQQ) is an ETF that tracks the Nasdaq-100 Index, which is composed of the 100 largest non-financial companies that trade on the Nasdaq market. As you might imagine, it's rather heavily weighted towards technology companies, though it omits the big ones that trade on the New York Stock Exchange, such as Salesforce.com (NYSE: CRM), IBM (NYSE: IBM) and Oracle (NYSE: ORCL). As an index fund, it sports a low annual fee -- 0.20%. Considering that many non-index mutual funds charge 1% or more, this is a terrifically low fee (though some index funds charge even less). Low fees mean that you get to keep most of the fund's gains, instead of handing over a meaningful chunk to the fund company. QQQ holdings Here are the fund's top 10 holdings, as of early March 2021: Company Percent of ETF's Assets Apple 11.44% Microsoft 9.44% Amazon.com 8.31% Tesla 4.44% Alphabet Class C 3.62% Facebook 3.35% Alphabet 3.28% NVIDIA 2.75% PayPal 2.57% Intel (NASDAQ: INTC) 2.07% Data source: Morningstar.com. With names like that in its roster, you might suspect that the fund has a good track record -- and you'd be right. Invesco notes that the ETF has been rated the best performer among large-cap growth funds, citing Lipper total return stats as of Dec. 31, 2020. The QQQ ETF is terrific if you'd like to own chunks of roughly a hundred companies like the ones above, but understand that most of your investment will be in just four companies: Apple, Microsoft, Amazon.com, and Google parent Alphabet. Together those companies represent about 36% of the fund's value. If the 10th-largest holding represents just 2.07%, you can imagine that not too much of your money will be in holdings 99 or 100. Still, the oversized holdings are in companies that have been growing like gangbusters in recent years and may well keep doing so. Image source: Getty Images. A smaller alternative: the QQQJ If you like the idea of the QQQ for your long-term portfolio but you'd also like some exposure to smaller companies, consider parking some of your money in the Invesco NASDAQ Next Gen 100 ETF (NASDAQ: QQQJ). As the fund company explains, the ETF "is based on the NASDAQ Next Generation 100 Index (Index) [and invests] at least 90% of its total assets in the securities that comprise the Index by investing in the 101st to the 200th largest companies on the NASDAQ." So it's chock-full of large-cap and mid-cap non-financial companies, including many with a technology focus. Here's a recent list of its top holdings: Company Percent of ETF's Assets Roku 2.69% CrowdStrike 2.41% ViacomCBS 2.21% The Trade Desk 2.02% Etsy 1.79% Zscaler 1.65% Fortinet 1.63% Zebra Technologies 1.62% DraftKings 1.57% Old Dominion Freight Line 1.50% Data source: Morningstar.com. One thing to note is that this fund is far less concentrated in its top holdings. Its annual fee is also a bit lower, at 0.15%. Most of the companies in both top-10 lists have performed very strongly over the past year or so, leading some to worry that they may be overvalued and are more likely to retract a bit in the near future than keep surging. No one can know what any stock will do in the short term, so these funds are best thought of as long-term portfolio candidates. And if you're worried about possible overvaluation, you might opt to just park some of your available money in them now, and reserve some money to buy additional shares later when they may be priced lower. Find out why Amazon is one of the 10 best stocks to buy now Motley Fool co-founders Tom and David Gardner have spent more than a decade beating the market. After all, the newsletter they have run for over a decade, Motley Fool Stock Advisor, has tripled the market.* Tom and David just revealed their ten top stock picks for investors to buy right now. Amazon is on the list -- but there are nine others you may be overlooking. Click here to get access to the full list! *Stock Advisor returns as of February 24, 2021 John Mackey, CEO of Whole Foods Market, an Amazon subsidiary, is a member of The Motley Fool's board of directors. Suzanne Frey, an executive at Alphabet, is a member of The Motley Fool's board of directors. Teresa Kersten, an employee of LinkedIn, a Microsoft subsidiary, is a member of The Motley Fool's board of directors. Randi Zuckerberg, a former director of market development and spokeswoman for Facebook and sister to its CEO, Mark Zuckerberg, is a member of The Motley Fool's board of directors. Selena Maranjian owns shares of Alphabet (A shares), Alphabet (C shares), Amazon, Apple, Facebook, IBM, Microsoft, PayPal Holdings, Salesforce.com, and The Trade Desk. The Motley Fool owns shares of and recommends Alphabet (A shares), Alphabet (C shares), Amazon, Apple, CrowdStrike Holdings, Inc., Etsy, Facebook, Microsoft, NVIDIA, Old Dominion Freight Line, PayPal Holdings, Roku, Salesforce.com, Tesla, The Trade Desk, Zebra Technologies, and Zscaler. The Motley Fool recommends Fortinet and Intel and recommends the following options: long January 2023 $57 calls on Intel, long January 2022 $1920 calls on Amazon, short January 2023 $57 puts on Intel, short January 2022 $1940 calls on Amazon, and long January 2022 $75 calls on PayPal Holdings. The Motley Fool has a disclosure policy.Source