Pericardial modification devices and methods can be used for the treatment of heart conditions. For example, this document describes devices and methods for treating heart failure with preserved ejection fraction, including diastolic heart failure, by performing a pericardial modification procedure. In one example embodiment, a method for treating diastolic heart failure includes creating an opening in a patient's pericardial tissue, or removing a portion of the patient's pericardial tissue. The creation of the opening or the removing of the pericardial tissue reduces pressure exerted by the pericardial tissue on the patient's heart. The pericardial modification procedures are performed while avoiding trauma to the epicardial surface of heart as well as the phrenic nerves.