The UNIX operating system was originally developed for use by programmers within an open systems environment. The adoption of UNIX as a common operating system across several different platforms has increased the need for security beyond its original purpose. As a result, many UNIX installations have less-than-optimal security. In addition, there are several versions of UNIX on the market today with differing security features. This chapter discusses basic access controls (e.g., passwords) and directory and file permissions within the UNIX system. The concepts discussed are applicable to all versions of UNIX unless specifically noted otherwise.