This guide was compiled to provide an aid for the rapid identification of the more common pest and non-pest thistle species in South Dakota. Because of the great expense of managing and controlling invasive thistles in rangelands, pastures, and croplands, it is essential to be accurate in the identification of any species at hand. The recognition of native species not only helps maintain local biodiversity, but also prevents wasted effort, controls costs, and minimizes herbicide use and chemical pollution.