The ascomycete fungus Clonostachys rosea was reported as an aggressive mycoparasite in the late 1950s (Barnett and Lilly, 1962), and initial attempts to use it for biological control of plant diseases soon followed (Shigo, 1958). Since then, there has been a wealth of new knowledge emerging concerning the ecology, physiology and genetics of C. rosea, as well as concerning its applied use as a biological control agent (BCA) including formulation, application strategy, efficiency and safety. In this chapter, we use the definition of biological control as the use of living organisms for the control of plant pathogens/diseases in line with the recent update on the terminology, where biological control falls under the umbrella ?bioprotection?, with the term BCAs being used only for living organisms, whereas products based on non-living, nature-based substances are another separate part of bioprotection (Stenberg et al., 2021). Due to the extensive literature available on C. rosea, this chapter does not represent a comprehensive review but rather aims to highlight selected aspects of C. rosea with respect to ecology, mechanisms of action, targeted crops and diseases and product development. Based on morphology, C. rosea (Link) Schroers, Samuels, Seifert & W. Gams was identified as the anamorph of the teleomorph Bionectria ochroleuca(Schwein.) Schroers & Samuels (Schroers et al., 1999). This was later confirmed based on DNA sequence data, including internal transcribed spacer (ITS) ribosomal DNA and ?-tubulin (tub) gene sequences (Schroers, 2001). Following the one-fungus, one-name principle, the use of C. rosea as the preferred species label was proposed due to its established use in the scientific literature (Rossman et al., 2013). Until 1999, strains of C. rosea were referred to as Gliocladium roseum Bainier, now considered a synonym that is sometimes still in use, especially in a more applied, biocontrol context. Two variants of C. rosea can be found in the literature, C. rosea forma (f.) rosea (G. roseum) and C. rosea f. catenulata (G. catenulatum), primarily distinguished by the colour of the conidia (white/yellow/salmon and green, respectively). However, a recent study using genealogical concordance phylogenetic species recognition indicates that the two variants constitute a single species (Moreira et al., 2016). Although the vast majority of reports of biological control of plant diseases involves the species C. rosea, there is evidence to suggest that certain strains from other, closely related species, also possess biocontrol properties (Table 1, Broberg et al., 2021; Sun et al., 2017; Krauss et al., 2006; García et al., 2003).Strains of C. rosea have been isolated from all continents except Antarctica and from a wide range of habitats (Sun et al., 2020a; Sutton et al., 1997), indicating a cosmopolitan distribution. Strains are typically isolated from soil, fungi, plant debris and from plant parts including roots, leaves and flowers (Walker and Maude, 1975; Nobre et al., 2005; Mueller and Sinclair, 1986; García et al., 2003), but isolations from nematodes and insects are also reported (Verdejo-Lucas et al., 2002; Haarith et al., 2020). Strains of C. rosea are even present as endophytes in several halophyte plant species in coastal areas (You et al., 2017). This habitat distribution should be viewed in light of the ecological generalist lifestyle of C. rosea, which includes plant endophytism, rhizosphere competence, polyphagous ability and mycoparasitism (Shigo, 1958; Li et al., 2002; Chatterton and Punja, 2012; Saraiva et al., 2015; Maillard et al., 2020). The traits that form the basis of the nutritional versatility that characterises generalist behaviour in C. rosea is tightly connected with its ability to control plant diseases and its use as a BCA.