Index funds are the simplest, lowest-cost way most people can invest in the stock market — and decades of evidence show they outperform the majority of professionally managed funds over the long term. Despite that record, many new investors skip them in favor of individual stocks or actively managed funds. Here’s what an index fund actually is, why they work, and how to start investing in one.
What Is an Index Fund?
An index fund is a type of investment fund designed to replicate the performance of a specific market index. A market index is a list of securities that represents a segment of the market — for example, the S&P 500 tracks the 500 largest publicly traded US companies.
When you invest in an S&P 500 index fund, you’re effectively buying a tiny slice of all 500 companies in proportion to their market size. When the index goes up, your fund goes up. When it falls, your fund falls.
This is called passive investing — the fund isn’t trying to pick winning stocks or outperform the market. It just tracks the index mechanically, which keeps costs extremely low.
Why Index Funds Work
Low fees compound in your favor
The expense ratio of a typical S&P 500 index fund is between 0.03% and 0.20% annually. An actively managed fund trying to beat the market typically charges 0.50%–1.50%. On $100,000, the difference between 0.05% and 1.00% is $950/year. Compounded over 30 years, that gap translates to hundreds of thousands of dollars.
Most active managers underperform
The SPIVA report (S&P Indices Versus Active) consistently shows that over 80–90% of actively managed US stock funds underperform their benchmark index over any 15-year period. Professional stock pickers, on average, fail to beat the market they’re trying to outperform — especially after fees.
Built-in diversification
An S&P 500 index fund holds 500 different stocks across every major sector — technology, healthcare, financials, consumer goods, energy, and more. If one company’s stock collapses, it represents a fraction of a percent of your investment. Diversification is the closest thing to a free lunch in investing.
Common Types of Index Funds
S&P 500 index funds
Track the 500 largest US companies. The most popular starting point for most investors. Examples: Vanguard S&P 500 ETF (VOO), Fidelity 500 Index Fund (FXAIX), iShares Core S&P 500 ETF (IVV).
Total stock market funds
Track the entire US stock market, including small and mid-size companies beyond the S&P 500’s large caps. Slightly broader diversification. Examples: Vanguard Total Stock Market ETF (VTI), Fidelity Total Market Index Fund (FSKAX).
International index funds
Track stocks in developed or emerging markets outside the US. Adding international exposure reduces your dependence on the US economy. Examples: Vanguard Total International Stock ETF (VXUS), iShares Core MSCI Total International Stock ETF (IXUS).
Bond index funds
Track a bond market index, providing lower-volatility income and a counterweight to stock market swings. Examples: Vanguard Total Bond Market ETF (BND), Fidelity US Bond Index Fund (FXNAX).
Sector index funds
Track a specific industry sector — technology, healthcare, real estate, energy, etc. Higher concentration risk than broad market funds. Use sparingly and intentionally.
Index Fund vs. ETF: What’s the Difference?
This is a common source of confusion. Most index funds come in two forms:
- Mutual fund form: Purchased at end-of-day net asset value (NAV), often directly from the fund company (Vanguard, Fidelity). Minimum investment may apply.
- ETF (exchange-traded fund) form: Traded throughout the day on a stock exchange like any stock. Usually no minimum investment; can buy fractional shares at most brokerages.
For most investors the distinction is minor. ETFs often have slightly lower expense ratios and greater flexibility, but both forms deliver index exposure at low cost.
How to Start Investing in Index Funds
Step 1: Open a brokerage account
Fidelity, Vanguard, and Schwab all offer excellent index funds with no trading commissions. If investing for retirement, start with a Roth IRA or traditional IRA. If you’ve maxed your retirement accounts, open a taxable brokerage account.
Step 2: Pick your fund(s)
For simplicity, one of these options works for most beginners:
- One-fund solution: A target-date fund (e.g., Vanguard Target Retirement 2055) automatically diversifies across US stocks, international stocks, and bonds — and gradually shifts conservative as you approach retirement
- Two-fund: Total US market fund + total international fund
- Three-fund: Total US market + total international + total bond market
Step 3: Set up automatic contributions
Automate monthly deposits from your bank account and set the funds to automatically reinvest dividends. The less you have to think about it, the better.
Step 4: Rebalance once a year
After 12 months, check if your allocation has drifted more than 5–10% from target. Rebalance by selling what’s overweight and buying what’s underweight. Most target-date funds do this automatically.
Frequently Asked Questions
Can you lose money in an index fund?
Yes. Index funds fall when the market falls. The S&P 500 dropped 38% in 2008 and 34% in early 2020. Investors who stayed invested recovered within a few years in both cases. Index funds are not risk-free, but they are appropriate for long-term goals of 5+ years.
What’s the minimum to invest in an index fund?
ETF versions can be purchased as fractional shares starting around $1 at most major brokerages. Mutual fund minimums vary: Fidelity’s index mutual funds have no minimum; Vanguard mutual funds typically require $1,000–$3,000.
Are index funds good for beginners?
Yes — they’re arguably the best starting point. Low cost, broad diversification, no stock-picking required, and decades of strong long-term performance.
Bottom Line
Index funds are the cornerstone of nearly every sound long-term investment strategy. They’re cheap, diversified, and consistently outperform most alternatives over time. Pick a total market or S&P 500 fund, contribute regularly, and let compounding do the work.
Related: How to Build an Investment Portfolio from Scratch 2026.