Abstract: The bacterium Bacillus thuringiensis is the main source of insecticidal proteins in insect resistant plants. However, biochemical and genetic studies have shown that insect resistance to B. thuringiensis (Bt) toxins can occur and with the advent of Bt transgenic crops this is a major concern. Several insect species have shown resistance to these toxins in the laboratory but the diamondback moth, Plutella xylostella is the only species which has evolved resistance under field conditions to date. Many studies have been done to elucidate the mode of action of the toxins and the mechanisms and genetics of resistance. In this article Bt toxins, their mode of action, mechanisms and genetics of resistance and management strategies for delaying resistance are reviewed. The emphasis is placed on examining the presently recommended high dose/refuge strategy.