Biological control against plant diseases has been extensively studied in recent decades, leading to the identification and development of a significant number of biocontrol agents with various modes of action. Considerable scientific and industrial investments are needed for the development and the commercialization of these protection tools. One of the main issue for the adoption of biocontrol by farmers is the reliability and stability of its efficacy in field conditions including greenhouses. Efficacy of biocontrol agents (especially that of microbes) is managed by complex factors linked to the changeable environmental conditions encountered in the field (e.g. microclimatic variations) and to farming practices (e.g. compatibility with other control methods, plant fertirrigation). Efficacy is also linked to the biological properties of the biocontrol agent (e.g. ecological competence, quality of the products, mode of application, mechanism of action, persistence of its efficacy) and of the plant pathogen (e.g. type of disease, inoculum pressure, diversity of sensitivity). In this chapter, examples of biocontrol agents used against soil-borne and foliar diseases, their modes of action as well as the factors of their efficacy will be addressed.