The provision by merchant banks of a guaranteed market for a new issue of shares. Firms making new issues cannot know in advance whether there will be sufficient demand for the shares they offer at the issue price, from the public or institutional investors. An underwriter removes the uncertainty as to whether shares will sell by promising to buy any the market does not take up. These shares then have to be sold, which may involve accepting a low price or a long wait. A commission is charged for this service.