A stock exchange, securities exchange, or bourse is an exchange where stockbrokers and traders can buy and sell securities, such as shares of stock, bonds and other financial instruments. Stock exchanges may also provide facilities for the issue and redemption of such securities and instruments and capital events including the payment of income and dividends. Securities traded on a stock exchange include stock issued by listed companies, unit trusts, derivatives, pooled investment products and bonds. Stock exchanges often function as "continuous auction" markets with buyers and sellers consummating transactions via open outcry at a central location such as the floor of the exchange or by using an electronic system to process financial transactions.

To be able to trade a security on a particular stock exchange, the security must be listed there. Usually, there is a central location for record keeping, but trade is increasingly less linked to a physical place as modern markets use electronic communication networks, which give them advantages of increased speed and reduced cost of transactions. Trade on an exchange is restricted to brokers who are members of the exchange. In recent years, various other trading venues such as electronic communication networks, alternative trading systems and "dark pools" have taken much of the trading activity away from traditional stock exchanges.

Initial public offerings of stocks and bonds to investors is done in the primary market and subsequent trading is done in the secondary market. A stock exchange is often the most important component of a stock market. Supply and demand in stock markets are driven by various factors that, as in all free markets, affect the price of stocks (see stock valuation).

There is usually no obligation for stock to be issued through the stock exchange itself, nor must stock be subsequently traded on an exchange. Such trading may be off-exchange or over-the-counter. This is the usual way that derivatives and bonds are traded. Increasingly, stock exchanges are part of a global securities market. Stock exchanges also serve an economic function in providing liquidity to shareholders in providing an efficient means of disposing of shares. In recent years, as the ease and speed of exchanging stocks over digital platforms has increased, volatility in the day-to-day market has increased, too.

The beginnings of lending were in Italy in the late Middle Ages. In the 14th century, Venetian lenders would carry slates with information on the various issues for sale and meet with clients, much like a broker does today. Venetian merchants introduced the principle of exchanging debts between moneylenders; a lender looking to unload a high-risk, high-interest loan might exchange it for a different loan with another lender. These lenders also bought government debt issues. As the natural evolution of their business continued, the lenders began to sell debt issues to the first individual investors. The Venetians were the leaders in the field and the first to start trading securities from other governments, yet did not embark on private trade with India. Nor did the Italians connect on land with the Chinese Silk Road. Along the potential overland trade route, Holy Roman Emperor Frederick II repulsed advances by Mongol Batu Kahn (Golden Horde) in 1241. There is little consensus among scholars as to when corporate stock was first traded. Some view the key event as the Dutch East India Company's founding in 1602, while others point to much earlier developments (Bruges, Antwerp in 1531 and in Lyon in 1548). The first book in history of securities exchange, the Confusion of Confusions, was written by the Dutch-Jewish trader Joseph de la Vega and the Amsterdam Stock Exchange is often considered the oldest "modern" securities market in the world. On the other hand, economist Ulrike Malmendier of the University of California at Berkeley argues that a share market existed as far back as ancient Rome, that derives from Etruscan "Argentari". In the Roman Republic, which existed for centuries before the Empire was founded, there were societates publicanorum, organizations of contractors or leaseholders who performed temple-building and other services for the government. One such service was the feeding of geese on the Capitoline Hill as a reward to the birds after their honking warned of a Gallic invasion in 390 B.C. Participants in such organizations had partes or shares, a concept mentioned various times by the statesman and orator Cicero. In one speech, Cicero mentions "shares that had a very high price at the time". Such evidence, in Malmendier's view, suggests the instruments were tradable, with fluctuating values based on an organization's success. The societas declined into obscurity in the time of the emperors, as most of their services were taken over by direct agents of the state.

Tradable bonds as a commonly used type of security were a more recent innovation, spearheaded by the Italian city-states of the late medieval and early Renaissance periods.

Joseph de la Vega, also known as Joseph Penso de la Vega and by other variations of his name, was an Amsterdam trader from a Spanish Jewish family and a prolific writer as well as a successful businessman in 17th-century Amsterdam. His 1688 book Confusion of Confusions explained the workings of the city's stock market. It was the earliest book about stock trading and inner workings of a stock market, taking the form of a dialogue between a merchant, a shareholder and a philosopher, the book described a market that was sophisticated but also prone to excesses, and de la Vega offered advice to his readers on such topics as the unpredictability of market shifts and the importance of patience in investment.

In England, London's first stockbrokers were barred from the old commercial center known as the Royal Exchange, reportedly because of their rude manners, in the late 17th century. Instead, the new trade was conducted from coffee houses along Exchange Alley. By 1698, a broker named John Castaing, operating out of Jonathan's Coffee House, was posting regular lists of stock and commodity prices. Those lists mark the beginning of the London Stock Exchange.