Weeds, insects, rodents, and pathogens are major problems in agricultural and urban environments; there is a clear need to augment chemical methods of control with biological methods. Until now these efforts have had limited success because of insufficient virulence of the host-specific organisms used. Naturally occurring biological agents are in evolutionary balance with their hosts, and attaining the level of control typically desired would lead to extinction of both the control agent and its host.In this book, the main researchers involved in enhancing fungal, bacterial, virus and insect biological control agents on different targets review progress in overcoming the barrier of insufficient virulence. This multi-disciplinary group, with backgrounds in many facets of biotechnology and crop protection, reviews their work and that of others, and describes the approaches, the successes and the remaining barriers in an integrated manner.