Abstract: Directive 128/2009/EC on the Sustainable Use of Pesticides encourages EU Member States to promote low pesticide-input pest control by the implementation of tools for pest monitoring and decision making, and by giving priority to non-chemical methods, including the use of biological control agents (BCAs). Although the intensive research developed in the last decades the practical use of BCAs is still challenging. Biocontrol of plant diseases involves complex interactions between two living organisms (the pathogen and the BCA), the plant and the environment. To achieve effective disease management is then crucial having knowledge on: (i) the life cycle of both, the pathogen and the BCA; (ii) the mode of action of the BCA against the pathogen; and (iii) how the two are influenced by the environment. Mathematical models for pathogen and BCAs could help understanding these relationships and then contributing to optimisation of the BCA applications. In this work, two examples are presented on how the use of BCAs can be improved by using mathematical modelling: Ampelomyces spp. hyperparasites against Erysiphe necator (the grape powdery mildew fungus), and BCAs against Botrytis cinerea (causing Botrytis bunch rot in grapes).