Improved methods for forming gate isolation structures between portions of gate electrodes and semiconductor devices formed by the same are disclosed. In an embodiment, a method includes forming a channel structure over a substrate; forming a first isolation structure extending in a direction parallel to the channel structure; forming a dummy gate structure over the channel structure and the first isolation structure; depositing a hard mask layer over the dummy gate structure; etching the hard mask layer to form a first opening through the hard mask layer over the first isolation structure; conformally depositing a first dielectric layer over the hard mask layer, in the first opening, and over the dummy gate structure; etching the first dielectric layer to extend the first opening and expose the dummy gate structure; and etching the dummy gate structure to extend the first opening and expose the first isolation structure.