The article provides an overview of the insecticidal crystal proteins produced by *Bacillus thuringiensis* and their genes. These proteins, known as δ-endotoxins, are highly specific to certain insect species and have been used as biological insecticides for over two decades. The crystal proteins are released in the larval midgut, where they are proteolytically converted into toxic fragments that disrupt the osmotic balance of the midgut epithelial cells, leading to cell swelling and lysis. The article discusses the diversity and classification of crystal protein genes, highlighting four major classes based on their host range: Lepidoptera-specific (I), Lepidoptera- and Diptera-specific (II), Coleoptera-specific (III), and Diptera-specific (IV). It also covers the expression of these genes, the mode of action of the toxins, and the factors that determine their insect specificity. The article concludes with a discussion on the use of *B. thuringiensis* crystal proteins in plant protection, including the development of transgenic plants expressing these toxins to create insect-resistant crops. Despite the widespread use of *B. thuringiensis* formulations, no cases of resistance development in the field have been reported, although one case of resistance in a laboratory setting has been documented.The article provides an overview of the insecticidal crystal proteins produced by *Bacillus thuringiensis* and their genes. These proteins, known as δ-endotoxins, are highly specific to certain insect species and have been used as biological insecticides for over two decades. The crystal proteins are released in the larval midgut, where they are proteolytically converted into toxic fragments that disrupt the osmotic balance of the midgut epithelial cells, leading to cell swelling and lysis. The article discusses the diversity and classification of crystal protein genes, highlighting four major classes based on their host range: Lepidoptera-specific (I), Lepidoptera- and Diptera-specific (II), Coleoptera-specific (III), and Diptera-specific (IV). It also covers the expression of these genes, the mode of action of the toxins, and the factors that determine their insect specificity. The article concludes with a discussion on the use of *B. thuringiensis* crystal proteins in plant protection, including the development of transgenic plants expressing these toxins to create insect-resistant crops. Despite the widespread use of *B. thuringiensis* formulations, no cases of resistance development in the field have been reported, although one case of resistance in a laboratory setting has been documented.