TeamCity logs user actions into the Audit log, which is available at the Administration | Audit page. Here you can find who deleted a build configuration or project, assigned a role to a user, added a user to a group, modified a build configuration and much more. To find needed information faster, filter the log by the activity type, build configurations and/or particular users.

If project or build configuration settings were modified, you can not only see the name of user who made the modification, but also view the change itself by clicking corresponding link.

Since project and build configuration settings are stored on disk in plain xml, the link will just open the usual TeamCity diff window showing changes in these xml files.

You can also see the latest modifications made to a project or build configuration right from project/build configuration settings page.

Audit log also can be retrieved in a text form, see {{logs\teamcity-activities.log }} log file.