Methods and apparatus for providing a security check are disclosed. Many conventional security checks require the recording or transmission of a user data sets associated with respective authorised users. A problem arises with these systems if the user data set becomes known by unauthorised persons. In order to address this problem the present invention proposes the recording or transmission of only part of the user data set in insecure sections of the system. For example, a security check preventing unauthorised remote access to a shared computer (20) is provided by capturing a user data set representing a user's iris, operating the user's personal computer (10) to select only a portion of the captured data set requested by the shared computer (20) and transmitting the portion along a telecommunications line (16).; The shared computer (20) derives partial data sets from stored user data sets using a similar selection to that used by personal computer (10) and compares the partial data set it has derived with the partial data set transmitted by the personal computer (10) in deciding whether to grant access. Point-of-sale devices and cards recording only partial user data sets are also disclosed.