Backflow, or the reverse flow of contaminated water into a potable water distribution system, is caused by backsiphonage or backpressure. State regulations require all backflow prevention assemblies to be inspected and tested by certified testers. The list of certified backflow prevention assembly testers provided below is not intended as an endorsement, certification or warranty to any company or product by or from the Missouri Department of Natural Resources. For more information about how to become a tester, see How to Become a Certified Backflow Tester. For general information about backflow in a potable water distribution system and backflow prevention, see Backflow Prevention.