Determining if a weed control failure is due to resistance or some other factor is an investigational process that involves asking a lot of questions in order to rule things out. It’s important to remember that weed control failures are usually not an indication of resistance development. Weed control failures can and do occur from multiple and interacting factors including weather and application errors. Resistance should not be assumed to be the cause of a weed control failure; other reasons must be investigated first. This paper presents information that can help you determine if herbicide resistance should be suspected for a weed control failure, or if it is due to other factors.