Proximal Attachment
Iliocostalis thoracis arises from the upper borders of the angles of the seventh to twelfth ribs, medial to iliocostalis lumborum.  

Distal Attachment
It ascends to the superior borders of the angles of the first to sixth ribs and the posterior surface of the transverse process of C7.

Nerve Supply
Iliocostalis thoracis is supplied by the dorsal rami of T1 to T12.

Action
When working on one side only, it produces lateral flexion and some extension.  Both sides work together to extend the thoracic spine.