When oil spills occur in U.S. waters, federal law places primary liability on the vessel owner or operator--that is, the responsible party--up to a statutory limit. As a supplement to this "polluter pays" approach, a federal Oil Spill Liability Trust Fund administered by the Coast Guard pays for costs when a responsible party does not or cannot pay. The Coast Guard and Maritime Transportation Act of 2006 directed GAO to examine spills that cost the responsible party and the Fund at least $1 million. This report answers three questions: (1) How many major spills (i.e., $1 million or more) have occurred since 1990, and what is their total cost? (2) What factors affect the cost of spills? and (3) What are the implications of major oil spills for the Oil Spill Liability Trust Fund? GAO's work to address these objectives included analyzing oil spill costs data, interviewing federal, state, and private-sector officials, and reviewing Coast Guard files from selected spills.