This presentation will begin with an introduction to the concepts of "owner", "group", and "other" as well as "read", "write", and "execute" permissions. We will discuss how these permissions affect files and directories differently. With real examples, we will see how complications and undesired access can arise from different permission settings. A survey of common permissions schemes will be made. The SUID, SGID, and the "sticky bit" will be introduced as means of surmounting common security issues in collaborative directories. With a firm conceptual foundation in place, Access Control Lists can be discussed as a means of fine-tuning access granted by standard permissions. The presentation will close with a dicsusssion of how to implement, manage, and determine ACLs.