Access control is perhaps the most basic aspect of computer security. Nearly all applications that deal with financial, privacy, safety, or defense include some form of access control. In many systems access control takes the form of a simple password mechanism, but many require more sophisticated and complex control. In addition to the authentication mechanism (such as a password), access control is concerned with how authorizations are structured. In some cases, authorization may mirror the structure of the organization, while in others it may be based on the sensitivity level of various documents and the security level of the user accessing those documents. This publication explains some of the most commonly used access control services available in information technology systems, their structure, where they are likely to be used, and advantages and disadvantages of each.