Applying Biological Control Agents (BCAs) appears to be a promising and eco-friendly approach for the management of many plant pathogens. In the Pythium genus, some mycoparasitic species such as P. acanthicum, P. lycopersicum, P. mycoparasitic, P. nunn, P. oligandrum and P. periplocum have been effectively used to control wide range of plant pathogenic fungi. P. oligandrum generates two types of elicitor, possibly released by the oomycete or perhaps purified from the cell wall. These two elicitors, e.g. oligandrin and cell wall proteins (CWPs), are specific to this oomycete. Combining both oligandrin and CWPs, capable of activating more genes, could offer an ability to enhance plant health. The aim of the current chapter is to focus on the most important Pythium species used as a biocontrol agent.