Biological control is a key process in sustainable agriculture, pesticide reduction, and in transformation of agriculture. To design agroecosystems that support natural enemies of pests, comprehensive information about their biology, including information on what kinds of plant-based food resources, alternate hosts, and shelter or nesting sites must be provided. This book reviews parasitic and predacious insects as natural enemies of phytophagous insects and mites. It also provides information on phytophagous insects as natural enemies of invasive weeds. The book focuses on aspects of beneficial insects as biological control agents and how humans benefit directly and indirectly from them.