This chapter focuses primarily on insecticides based on viable microorganisms. Other biological insecticides such as plant extracts (e.g. nicotine, azadirachtin) and purified fermentation products (e.g. avermectins and spinosins) are covered in other chapters. Bacillus thuringiensis is the most widely used microbial pesticide with uses both as a viable organism and with its insecticidally active proteins incorporated into genetically modifiedcrop plants. The proteins are activated inside the alkaline insect gut and should present minimal risks to mammals. Granulovirus and nuclear polyhedrosis virus preparations are based on these naturally occurring arthropod specific pathogens. Beauvaria bassiana species are active against a range of insects and act by damaging the cuticle, invading and growing in the insect or possibly secreting a toxic secondary metabolite.