Described here are systems and methods for affecting tissue within a body to form a lesion. Some systems comprise tissue-affecting devices, devices that guide the advancement of the tissue-affecting elements to a target tissue region, devices that locate and secure tissue, and devices that help position the tissue-affecting devices along the target tissue. The methods described here comprise advancing a first tissue-affecting device to a first surface of a target tissue, advancing a second tissue-affecting device to a second surface of the target tissue, and positioning the first and second devices so that a lesion may be formed in the tissue between them. In some variations, the devices, systems, and methods described here are used to treat atrial fibrillation by ablating fibrillating tissue from an endocardial surface and an epicardial surface of a heart. Methods of closing, occluding, and/or removing the left atrial appendage are also described.
