An index fund is one of the simplest and most popular ways to invest. Instead of paying a manager to pick winners, an index fund simply buys everything in a market index — like the S&P 500 — and aims to match its return. The result is low fees, instant diversification, and a hands-off approach that has served long-term investors well.
What an index fund actually is
A market index measures the performance of a group of investments — the S&P 500 tracks about 500 of the largest U.S. companies, for example. An index fund holds the same investments as that index, in the same proportions, so its return closely follows the index itself.
Because the fund isn’t trying to beat the market — just match it — there’s no expensive research team to pay for. That’s why index funds typically charge very low fees, which is one of their biggest long-term advantages.
Why investors love them
- Low cost. Small fee differences compound into large sums over decades.
- Diversification. One fund can spread your money across hundreds or thousands of companies, so no single company sinks you.
- Simplicity. You don’t have to pick individual stocks or time the market.
Index funds vs. ETFs
Index funds come in two wrappers:
- Index mutual funds trade once per day at the closing price and sometimes require a minimum investment.
- Index ETFs (exchange-traded funds) trade like a stock throughout the day and often let you start with the price of a single share.
The underlying strategy can be identical — the choice usually comes down to minimums, how you like to buy, and what your brokerage offers.
How to start
- Open an account — a brokerage account, an IRA, or your workplace 401(k).
- Pick a broad, low-cost index fund (a total-market or S&P 500 fund is a common starting point).
- Invest regularly and let compounding work over time.
Compare beginner-friendly platforms in our guide to the best investing apps for beginners, see how steady investing grows with the Compound Interest Calculator, and decide where to hold your funds with Roth vs. Traditional IRA.
The bottom line
An index fund lets you own a slice of the whole market at rock-bottom cost, with built-in diversification and almost no maintenance. It won’t beat the market — by design — but for most long-term investors, reliably matching it at low cost is a winning strategy. Just remember that all investing carries risk, including the possible loss of principal.
Frequently asked questions
Are index funds a good investment for beginners?
What's the difference between an index fund and an ETF?
Can I lose money in an index fund?
Sources
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