An index fund is a collection of stocks or bonds that mirrors a list of companies or securities that already exists

An index fund does not pick winners. Instead, it holds every stock (or bond, or other security) that belongs to a specific index — a published list maintained by a financial company. The most common index is the S&P 500, which lists 500 large U.S. companies. An S&P 500 index fund holds all 500 of those companies in the same proportions the index does. When you buy shares in that fund, you own a tiny piece of all 500 companies at once.

The fund manager's job is not to decide which stocks are good. It is to track the index as closely as possible — to own what the index owns, in the same amounts, and to keep costs low. This is different from an actively managed fund, where a manager picks and chooses which stocks to buy, trying to beat the index. Index funds aim to match the index, not beat it.

Index funds exist for almost every major index. Some track the S&P 500. Others track the total U.S. stock market, international stocks, bonds, real estate, or combinations of these. The index itself is just a list; the fund is the investment product you can buy into.

Key Takeaways

  • An index fund holds all the securities in a published index in the same proportions, rather than picking individual winners and losers.
  • The fund manager's goal is to match the index's performance closely, which usually means keeping costs and trading activity low.
  • Index funds exist for hundreds of different indexes covering U.S. stocks, international stocks, bonds, real estate, and other asset types.
  • Because index funds do not require active stock-picking, they typically charge lower fees than actively managed funds.
  • The index itself changes over time as companies are added or removed, and the fund automatically adjusts its holdings to stay aligned.

How an index fund tracks its index

When you own an index fund, the fund manager buys and holds the exact stocks (or bonds) that belong to the index. If the S&P 500 index adds a new company or removes one, the fund does the same. The manager rebalances the fund's holdings to match the index's weightings — meaning if Apple makes up 7% of the index, Apple should make up roughly 7% of the fund.

This tracking is not perfect. The fund may hold a small amount of cash for incoming deposits or outgoing withdrawals. It may lag slightly behind the index because of trading costs and fees. This difference is called tracking error, and it is usually small — often less than 0.1% per year for large, well-run index funds.

The index itself is maintained by the company that created it. S&P Dow Jones Indices maintains the S&P 500. MSCI maintains several international indexes. Nasdaq maintains the Nasdaq-100. These companies publish the rules for which companies belong in each index and when they change. The fund manager follows those rules.

Common indexes and what they hold

The S&P 500 is the most widely tracked index in the United States. It includes 500 large U.S. companies chosen by S&P Dow Jones Indices. These are established companies like Microsoft, Coca-Cola, and JPMorgan Chase. An S&P 500 index fund gives you exposure to large-cap U.S. stocks.

The Total Stock Market Index is broader. It includes not just the 500 largest companies but thousands of mid-size and smaller U.S. companies. The most common version is the Wilshire 5000 Total Market Index, though the name is historical — it now includes more than 5,000 stocks. A total market index fund holds a wider slice of the U.S. economy.

The Nasdaq-100 focuses on 100 large non-financial companies, many in technology. It is more concentrated than the S&P 500 and skews toward growth sectors. An index fund tracking this index will have a heavier technology weighting.

International indexes include the MSCI EAFE (Europe, Australasia, Far East) and the MSCI Emerging Markets index. These track stocks outside the United States. Bond indexes include the Bloomberg U.S. Aggregate Bond Index, which holds thousands of U.S. bonds across different maturities and credit qualities.

Why costs matter in index funds

Because index funds straightforward hold what the index holds, the main difference between one S&P 500 index fund and another is cost. The fund charges a fee, called an expense ratio, expressed as a percentage of your investment per year. A fund with a 0.03% expense ratio charges $3 per year on every $10,000 you invest. A fund with a 0.50% expense ratio charges $50 on the same $10,000.

Over decades, this difference compounds. If two index funds track the same index but one charges 0.03% and the other charges 0.50%, the cheaper fund will deliver more of the index's returns to you. The difference grows larger the longer you hold the investment. This is why many investors compare expense ratios when choosing between index funds.

Index funds typically charge less than actively managed funds because the manager is not paying analysts to research stocks or trading frequently to try to beat the market. The manager straightforward buys and holds the index. This lower cost is one of the main reasons index funds have become popular.

Index funds versus actively managed funds

An actively managed fund employs a manager or team to pick individual stocks they believe will outperform the market. They buy some stocks and avoid others. They may trade frequently, trying to time the market or capitalize on short-term moves. This research and trading activity costs money, which is why actively managed funds typically charge higher fees — often 0.5% to 2% or more per year.

An index fund does not try to beat the market. It aims to match the market by holding the index. Because it does not require active stock-picking or frequent trading, it costs less to run. Over long periods, the lower costs of index funds often result in better returns to the investor, even though the fund is not trying to outperform.

Some actively managed funds do outperform their index over certain periods. But studies show that most do not consistently beat their index over many years after accounting for fees. This is why index funds appeal to investors who want broad market exposure at low cost without betting on a manager's skill.

How index funds fit into a portfolio

Index funds are often used as the core holding in a diversified portfolio. An investor might hold an S&P 500 index fund for U.S. stock exposure, an international index fund for non-U.S. stocks, and a bond index fund for fixed income. Together, these three index funds provide broad exposure across different asset classes and geographies with low costs.

Index funds can also be held inside retirement accounts like 401(k)s and IRAs, or in regular taxable brokerage accounts. The account type determines the tax treatment of gains and withdrawals, but the index fund itself works the same way in any account.

Some investors use index funds as their entire portfolio. Others combine index funds with individual stocks, actively managed funds, or other investments. Index funds are flexible enough to fit many different investment approaches.

Frequently Asked Questions

Do I own the actual stocks in an index fund?

Yes. When you buy shares of an index fund, you own a fractional stake in every stock the fund holds. If the fund owns 500 stocks, you own a tiny piece of all 500. You do not own them directly — the fund holds them on your behalf — but you have a legal claim to your proportional share of the fund's holdings.

What happens when a company is added to or removed from an index?

The index maintainer (like S&P Dow Jones Indices) announces the change. The fund manager then buys or sells shares to match the new index composition. This usually happens on a set date. Investors in the fund do not need to do anything — the fund adjusts automatically.

Can an index fund lose money?

Yes. An index fund's value rises and falls with the stocks or bonds it holds. If the S&P 500 drops 20% in a market downturn, an S&P 500 index fund will also drop roughly 20%. Index funds are not may provide investments. Over long periods, stock indexes have historically risen, but short-term losses are common.

How often should I check my index fund's performance?

Index funds are designed for long-term holding, so frequent checking can lead to emotional decisions during market swings. Many investors check their holdings quarterly or annually rather than daily. The fund's performance will track the index it follows, so checking the index's performance tells you roughly how your fund is doing.

Are all index funds the same if they track the same index?

They aim to track the same index, but differences exist. Expense ratios vary. Some funds use sampling (holding most but not all index stocks) while others hold every stock. Tax efficiency can differ. Trading costs and minimum investments vary by provider. Comparing expense ratios and fund details helps you choose between funds tracking the same index.