The Securities Exchange Act of 1934 (also called the Exchange Act, '34 Act, or 1934 Act) (Pub. L. 73–291, 48 Stat. 881, enacted June 6, 1934, codified at 15 U.S.C. § 78a et seq.) is a law governing the secondary trading of securities (stocks, bonds, and debentures) in the United States of America. A landmark piece of wide-ranging legislation, the Act of '34 and related statutes form the basis of regulation of the financial markets and their participants in the United States.

The 1934 Act also established the Securities and Exchange Commission (SEC), the agency primarily responsible for enforcement of United States federal securities law.

Companies raise billions of dollars by issuing securities in what is known as the primary market. Contrasted with the Securities Act of 1933, which regulates these original issues, the Securities Exchange Act of 1934 regulates the secondary trading of those securities between persons often unrelated to the issuer, frequently through brokers or dealers. Trillions of dollars are made and lost each year through trading in the secondary market.

One area subject to the 1934 Act's regulation is the physical place where securities (stocks, bonds, notes of debenture) are exchanged. Here, agents of the exchange, or specialists, act as middlemen for the competing interests in the buying and selling of securities. An important function of the specialist is to inject liquidity and price continuity into the market. Some of the more well known exchanges include the New York Stock Exchange, the NASDAQ and the NYSE American.

The 1934 Act also regulates broker-dealers without a status for trading securities. A telecommunications infrastructure has developed to provide for trading without a physical location. Previously these brokers would find stock prices through newspaper printings and conduct trades verbally by telephone. Today, a digital information network connects these brokers. This system is called NASDAQ, standing for the National Association of Securities Dealers Automated Quotation System.

In 1938, the Exchange Act was amended by the Maloney Act, which authorized the formation and registration of national securities associations. These groups would supervise the conduct of their members subject to the oversight of the SEC. The Maloney Act led to the creation of the National Association of Securities Dealers, Inc. – the NASD, which is a Self-Regulatory Organization (or SRO). The NASD had primary responsibility for oversight of brokers and brokerage firms, and later, the NASDAQ stock market.

In 1996, the SEC criticized the NASD for putting its interests as the operator of NASDAQ ahead of its responsibilities as the regulator, and the organization was split in two, one entity regulating the brokers and firms, the other regulating the NASDAQ market. In 2007, the NASD merged with the NYSE (which had already taken over the AMEX), and the Financial Industry Regulatory Authority (FINRA) was created.

In the last 30 years,^([timeframe?]) brokers have created two additional systems for trading securities. The alternative trading system, or ATS, is a quasi exchange where stocks are commonly purchased and sold through a smaller, private network of brokers, dealers, and other market participants. The ATS is distinguished from exchanges and associations in that the volumes for ATS trades are comparatively low, and the trades tend to be controlled by a small number of brokers or dealers. ATS acts as a niche market, a private pool of liquidity.

Reg ATS, an SEC regulation issued in the late 1990s, requires these small markets to 1) register as a broker with the NASD, 2) register as an exchange, or 3) operate as an unregulated ATS, staying under low trading caps.

A specialized form of ATS, the Electronic Communications Network (or ECN), has been described as the "black box" of securities trading. The ECN is a completely automated network, anonymously matching buy and sell orders. Many traders use one or more trading mechanisms (the exchanges, NASDAQ, and an ECN or ATS) to effect large buy or sell orders – conscious of the fact that overreliance on one market for a large trade is likely to unfavorably alter the trading price of the target security.