When physicians practice defensive medicine by ordering extra tests and procedures that have little or no benefit mainly to avoid malpractice lawsuits, they reduce the quality of the care they provide and increase health care costs. The introduction of reliable clinical practice guidelines-recommendations for optimizing patient care based on scientific evidence-promises to set standards of good care and reduce utilization of unnecessary procedures. Some have proposed that following such a guideline should give caregivers a liability "safe harbor," shielding them from any malpractice claim for failing to provide services not included in the guideline. The intent is to reduce incentives to practice defensively without imposing broader limits on liability through "tort reform." This brief examines how legislating safe harbors would work in theory and practice. It suggests that quality-promoting guidelines hold some promise for cutting wasteful defensiveness, but that practical feasibility limits their reach.