Access control lists (ACLs) provide flexible permissions for file systems beyond standard user/group/other permissions. ACLs allow specifying permissions for individual users or groups on each file and directory. They are enabled by default on Linux file systems like ext4 and XFS. Viewing ACLs with getfacl shows user, group, mask, and other permissions. The mask limits permissions for named users and groups. Setting and deleting ACLs uses setfacl commands. Default ACLs on a directory automatically apply to new files and subdirectories.