Abstract: Physical or electronic objects are encoded with identifiers, which serve to trigger object– appropriate responses from computer systems that encounter such objects The encoding may be steganographic (e g by digital watermarks), so the presence of such identifiers is not evident to persons encountering the objects. An exemplary application is a computer system that looks at a printed magazine advertisement (20) and initiates a link to a corresponding internet page In one such implementation, the computer system senses an identifier encoded in the advertisement, forwards the identifier to a remote database, receives from the database (17) a corresponding internet address (18a, 18b, 18c), and directs a browser to that address (18a, 18b, 18c) The same arrangement can be used for on–line ordering from printed merchandise catalogs.