When-issued (WI) trading is a type of securities transaction that allows investors to buy or sell a security before it is issued. When-issued securities transactions are generally offered when a company is launching a new security, completing a merger, or announcing a spinoff. The transaction is conditional and so there is no guarantee that it will be completed, particularly if the offering is canceled.

When-issued orders are made conditionally because the security has yet to be issued, and so in order to ensure the safety of investors, no trades can be made until the security has been issued. This allows those interested in buying or selling a security before its issue to commit to doing so at a certain price ahead of time and with the knowledge that the transaction will be completed.

Typically, when-issued trading takes place after the launching of a new security has been announced, but before the security is issued. The offering's sponsor will often decide to limit the amount of pre-issuance trading that occurs and may suspend when-issued orders if the offering is oversubscribed or generates more interest than expected. By offering when-issued transactions, the sponsors can gain an indication of the potential demand for their new security, and therefore better determine the success of their offering before committing to issuing it.

When-issued trading can help to provide liquidity to the secondary markets and prevent the price of a new security from gapping after it is issued. With when-issued trading, investors can engage in the pre-issuance speculation of a security's price. This involve a high degree of risk and so should only be done by experienced investors, or those willing to accept losses if the security does not perform as expected.

The primary example of when-issued trading are Treasury securities, as these are often traded with an assurance from the U.S. Treasury that they will be issued, but not necessarily with an exact date as to when they will be issued. Additionally, stock splits and other new issues of stocks and bonds are also commonly traded on a when-issued basis.

Overall, when-issued trading is a type of transaction that allows investors to buy or sell a security before it is issued. By offering when-issued transactions, the sponsors can gain an indication of the potential demand for their new security. This helps to create liquidity in the markets while also helping investors engage in the pre-issuance speculation of a security's price, at their own risk.