<p>The S&P 500, or Standard & Poor's 500 Index, is a widely recognized stock market index that tracks the performance of 500 leading companies listed on the <a href="/NewSite/encyclopedia/nasdaq/" class="ency-link">NASDAQ</a> or New York Stock Exchange. These companies are selected based on market capitalization, liquidity, and industry representation, providing a broad-based measure of the U.S. stock market.</p>

<h2>How It Works</h2>
<p>The S&P 500 is a market-capitalization-weighted index, meaning that each company's influence on the index is proportionate to its market capitalization, or the total value of its outstanding shares. This ensures that larger, more influential companies have a greater impact on the index's performance. The index is float-adjusted, meaning that only shares available for public trading are included in the calculation. The S&P 500 is rebalanced quarterly to ensure that it remains representative of the U.S. stock market.</p>

<h2>Why It Matters</h2>
<p>The S&P 500 is one of the most closely watched indices in the world, serving as a benchmark for the U.S. stock market and a barometer for the health of the broader economy. It is often used as a reference point for investors, fund managers, and analysts to evaluate the performance of other investments. The S&P 500 also serves as the underlying asset for a wide range of financial instruments, including exchange-traded funds (ETFs), options, and futures, providing investors with numerous ways to gain exposure to the U.S. stock market.</p>