The book chapter delves into the multifaceted world of Clonostachys, a group of free-living saprophytic ascomycete fungi that pervade diverse natural habitats, from soil to decomposing organic matter. With their remarkable potential as biocontrol agents against seed and soil-borne pathogens, Clonostachys species has reaped significant attention. These fungi exhibit fascinating parasitic and symbiotic relationships with both plants and animals, making them ubiquitous inhabitants of terrestrial and aquatic environments, while also offering a wide array of applications in agriculture. A distinguishing feature of Clonostachys species is their capacity to produce a diverse range of secondary metabolites, boasting potent antifungal, antibacterial, entomopathogenic, and nematicidal properties that can combat a multitude of plant pathogens and insects. In this chapter, we provide a comprehensive overview of the biology, phylogeny, diversity, biocontrol activities, and metabolites of Clonostachys species, shedding light on their significant contributions to the fields of agriculture.