During plant–pathogen interactions, plants mount sophisticated defenses to ward off their enemies. Plant defenses can be triggered by biocontrol agents. In recent years, various biocontrol agents have been defined as systemic resistance inducers of plant immunity. Trichoderma spp. are opportunistic plant colonizers that trigger host-induced systemic resistance (ISR), typically eliciting rapid defense responses that include massive transcriptional reprogramming. Root and leaf colonization by Trichoderma can prime plant defense, enabling robust plant responses to subsequent pathogen challenges. The onset of host immune responses can also trigger systemic acquired resistance (SAR) in tissues distal to the infection site. Trichoderma spp. are potent inducers of plant ISR. Systemic priming leading to rapid defense responses upon pathogen attack is common to both SAR and ISR. In this chapter, we will outline the current knowledge of plant immunity, emphasizing mechanisms underlying induced systemic resistance triggered by Trichoderma.