Botrytis cinerea is a necrotrophic fungal pathogen causing disease in many plant species leading to economically important crop losses. So far, fungicides are widely used to control this pathogen. However, in addition to their detrimental effects on the environment and potential risks for human health, increasing fungicide resistance has been observed in the B. cinerea population. Biological control, implying the application of microbial organisms to reduce disease, has gained importance as an alternative or complementary approach to fungicides. In this respect, the genus Trichoderma constitutes a promising pool of organisms with potential for B. cinerea control. In the first part we review the specific mechanisms involved in the direct interaction between the two fungi, including mycoparasitism, the production of antimicrobial compounds and enzymes (collectively called antagonism) and competition for nutrients and space. In addition, biocontrol has also been observed when Trichoderma was physically separated from the pathogen, thus implying an indirect systemic plant defence response. Therefore, in the second part we describe the consecutive steps leading to induced systemic resistance (ISR), starting with the initial Trichoderma-plant interaction followed by the activation of downstream signal transduction pathways and ultimately the defence response resulting in ISR (ISR-prime phase). Finally, we discuss the ISR-boost phase, representing the effect of the ISR-priming by Trichoderma spp. on plant responses after additional challenge with B. cinerea.