Do agents sign contracts with clients?

Beginning Writer's Answer Book, edited by Jane Friedman

Some agents require a contract before they’ll do business with a writer. They feel that written agreements offer the agent protection, since the author doesn’t pay her any money until a contract is drawn up containing a clause that provides for payment to the agent. There are, however, agents who prefer not to have a written agreement with authors; it doesn’t mean an agent isn’t reputable—that’s just her working style.