Authentication is how a user is identified, typically through confidential information such as user name and password. Authentication ensures that users of hardware or software are who they say they are.
Set up authentication features such as a password system in your platform operating systems to ensure that users are who they say they are.
Ensure that your personnel use employee badges properly to enter the computer room.
For user accounts: use access control lists where appropriate; set time-outs for extended sessions; set privilege levels for users.