The **S&P 500** (Standard & Poor’s 500) is a major stock market index that tracks the stock performance of 500 of the largest publicly traded companies in the United States, representing about 80% of the total U.S. equity market capitalization[1][2]. Established initially as a 233-company index in 1923 by the Standard Statistics Company and expanded to 500 companies in 1957, it has become the premier benchmark for U.S. large-cap stocks and a key indicator of the overall health of the U.S. economy[4]. The index is capitalization-weighted, meaning companies with larger market values have greater influence on its performance. For example, as of late 2025, the ten largest companies—such as Nvidia, Microsoft, Apple, Alphabet, and Amazon—constitute roughly 38% of the index's market cap[1]. The S&P 500 covers all 11 sectors of the economy, providing broad diversification and a comprehensive snapshot of market trends[2][5]. In addition to serving as a market barometer, the S&P 500 is integral to economic forecasting, contributing to composite indicators like the Conference Board Leading Economic Index[1]. Its components are selected by a committee based on criteria such as market capitalization, liquidity, and sector representation[6]. Being included in the S&P 500 is prestigious and financially impactful for companies, as index funds tracking the S&P 500 must buy shares of newly added firms, often boosting their stock prices[2]. Today, the S&P 500 is maintained by S&P Dow Jones Indices, a division of S&P Global, and is widely used to benchmark investment portfolios, guide passive investing strategies, and evaluate firm performance[7][8]. Investors typically access the index through mutual funds or ETFs, gaining instant exposure to a broad swath of the U.S. market. Its role as a market proxy and investment vehicle makes it a cornerstone of bot