The generic term “bio-effector” refers to the wide range of living organisms and natural compounds that can be used as active agents in biological approaches for crop production. They provide components in bio-stimulants and other products that improve the ability of crops to metabolize nutrients. The chapter reviews the wealth of research on the mechanism of action, applications and efficacy of key bio-stimulants such as Bacillus species, Pseudomonas species, Trichoderma species and arbuscular mycorrhizal fungi (AMF).