Bacillus thuringiensis, known as Bt, is a spore-forming bacterium that occurs naturally in soil and that produces highly specific insecticidal proteins called Cry proteins. These proteins are stomach poisons that specifically affect insects. Today, Bt preparations are considered as the most effective, specific and environmentally-friendly bioinsecticides; they have been used as biological pesticides in agriculture, forestry and in human health for the elimination of vectors of diseases for more than 60 years and their implementation far exceeds other microbial agents such as fungi, protozoa or viruses. This review on the use of this entomopathogenic bacterium in crop protection is not intended to be a compilation of the results of all the investigations made in this field. Instead, it is an attempt to provide an overview of the major trends and developments of Bt for the control of agricultural insect pests and to describe the main approaches that have been used to improve this natural bioinsecticide. Bt-based insecticides are considered safe for mammals and birds, and are safer for non-target insects than conventional insecticides; they have become the most widely used microbial insecticides. However, Bt products have several limitations, such as a narrow activity spectrum, instability in rain and sunlight, and inefficiency against pest feeding on internal tissues of the plants. The first step towards improving Bt has involved the isolation of new strains with higher and broader insecticidal activity against targeted insect pests and the cloning of cry genes encoding new insecticidal crystal proteins. A second strategy was to increase the persistence of its toxins in the field by encapsulation in recombinant asporogenic Bt strains or other heterologous recombinant microbial hosts; this protected the toxins against UV degradation and had the advantage that the transgenic microorganisms released into the environment were non-viable. Bt has also become a key source of genes for transgenic expression to provide pest resistance in plants and in so-called genetically modified plants. The engineering of plants to express Bt cry genes has been especially helpful against pests that attack parts of the plant that are usually not well protected by conventional insecticide application. The potential effects on human health and the environment of the large-scale use of these Bt crops are also in the scope of this review.