Foliar plant pathogens pose a serious problem for global food security. Biological control is considered ecologically friendly and a key alternative in disease management. Several organisms are known to be antagonistic against foliar pathogens. In this chapter important foliar pathogens and antagonistic microorganisms, their distribution, ecology, biology and their modes of action are described. Many researches carried out worldwide during the last decades on efficacy evaluation of biological control agents with some convincing success stories are extensively reviewed, as well as the factors that encourage or hamper their development.