Trichoderma reesei is a filamentous ascomycete frequently used as production organism in biotechnology, but also a model organism for plant cell wall degradation, light response, and since recently, for some aspects of sexual development in fungi. Mating under laboratory condition was achieved relatively late for T. reesei, which was considered asexual for decades. A defect in female fertility of the most prominent isolate, QM6a, led to substantial research efforts towards determinants of fertility in T. reesei and to the discovery of ham5 being mutated and hence responsible for female sterility in QM6a. The pheromone system of T. reesei is special in that it comprises an h-type peptide pheromone precursor (HPP1), which assumes a-type function, but also has alpha-type characteristics. Regulation of sexual development and fertility in T. reesei is performed by photoreceptors, the MAP-kinase pathway, G-protein signaling, and the transcription factors SUB1 and VEL1. The latter two regulators are further important for chemical communication prior to and during sexual development. Crosstalk of regulation associated with sexual development was observed with enzyme production and secondary metabolism. Last but not the least, in recent years T. reesei also emerged as an appropriate model system for studying molecular processes associated with meiosis, such as Repeat-Induced-Point mutation (RIP), DNA-cytosine methyltransferases or interhomologue recognition, and DNA strand pairing.