Publisher Summary The chapter focuses on the classification and mode of action of the insecticidal proteins of Bacillus thuringiensis (Bt) or Bt toxin. Bt is a gram-positive, aerobic, endospore-forming bacterium. Bt toxin gene is classified into four major classes: cry I, cry II, cry III, and cry IV. Cry is a Bt gene which codes a parasporal inclusion protein that exhibits pesticide activity. Bt toxin genes are usually plasmid borne but also chromosomally located. The chapter discusses on various aspects of the bacterium such as Bt fermentation, biology and genetics, molecular biology, and mechanism of action. The chapter also reviews application of Bt toxin as biopesticide. Bt is the most popular biological control agent. The most effective delivery system for Bt toxins is the transgenic plant. The importance and features of transgenic plants are summarized. Strategies to screen new Bt strains/genes, expression of the toxin protein in transgenic microorganisms and plants and various resistance management strategies in agricultural systems are examined. The review puts emphasis on agricultural application of Bt.