Biological control can be defined into several categories of which two have special relevance for biological control of plant diseases, i.e. inoculation and conservation biological control. This chapter defines biological control, and provides some tips on how to select a good biological control agent, whose success depends on the target pathogen, the crop, the cropping system, the resident microbiota and the environment. It also describes the mechanisms involved in biological control, such as competition, antibiosis, hyperparasitism (i.e. mycoparasitism, when it relates to fungal-fungal interactions), induced plant disease resistance and microbial plant-growth promotion. The importance of understanding the plant host/disease cycle for optimal biological control is also emphasized.