What Is the S&P 500 and Why Does It Matter?

The Complete Answer

The S&P 500 is an index of 500 of the largest publicly traded US companies, chosen by a committee and weighted by market value. Because its members are so large, those 500 names represent roughly 80% of the total value of the US stock market, which is why it is treated as the headline gauge of "how stocks did."

It is weighted by market capitalization, so the biggest companies carry the most influence. Today the largest handful — names like Apple, Microsoft, Nvidia, and Amazon — make up a substantial share of the index, meaning their moves can swing the whole thing more than the hundreds of smaller members combined.

It matters to ordinary investors because it is the benchmark almost everything is measured against, and because it is cheap and easy to own. Funds like VOO, IVV, and SPY track it for as little as 0.03%, and that single purchase gives you a stake in 500 companies across every major sector.

Its long-term record is the reason for its reputation: roughly 10% average annual returns over the past century, with no 20-year period that lost money. The catch is concentration — it is 100% US large-cap, with little exposure to small companies or international markets, which is why many investors pair it with VXUS and a bond fund.