an exchange can list a company, for instance because its stock is already being traded via informal channels.
Stocks whose market value and/or turnover fall below critical levels may be delisted by the exchange.
Initial listing requirements usually include supplying a history of a few years of financial statements (not required for "alternative" markets targeting young firms); a sufficient size of the amount being placed among the general public (the free float), both in absolute terms and as a percentage of the total outstanding stock; an approved prospectus, usually including opinions from independent assessors, and so on.
This typically occurs when a company goes out of business, declares bankruptcy, no longer satisfies the listing rules of the stock exchange, has become a private company, has become a subsidiary after a merger or acquisition, or wants to reduce regulatory reporting complexities and overhead, or if the stock volumes on the exchange from which it wishes to delist are not significant.
[4] In the United States, securities which have been delisted from a major exchange for reasons other than going private or liquidating may be traded on over-the-counter markets like the OTC Bulletin Board or the Pink Sheets.