One straightforward method is by purchasing shares of an exchange-traded fund (ETF) that mirrors the performance of the S&P 500. These ETFs can be bought and sold just like stocks through a brokerage account, offering the flexibility of trading throughout the day at market prices. Mutual funds that track the S&P 500 may also be an option if they are offered in retirement accounts or through your broker. It should be noted, however, that mutual funds can only be bought and sold at the end of each trading day, unlike ETFs.