Abstract: Experience from 40-years´ work with biological control of fungal diseases onapples, carrots, strawberries, and turf grass will be presented. We also describe our researchunder controlled conditions on how temperature, nutrients and antagonist concentration affectinteractions between fungi with regard to biocontrol. The challenges in selecting antagoniststhat can be active under Nordic climate conditions will be addressed as well as the need tounderstand the complex interactions between the pathogen, the antagonist and the climate thataffect biological control. Biological control is the use of nature´s own defenses againstdiseases or, as Cook and Baker (1983) stated, “Biological control is the reduction of theamount of inoculum or disease-producing activity of a pathogen, accomplished by or throughone or more organisms other than man”. Biological control can be achieved by stimulation ofnatural microflora (which includes not killing the beneficial microflora with chemicalpesticides), by applying selected antagonists, or by ”keeping the plants happy” with goodmanagement practices. In this paper we will describe five of our own case studies as well aslaboratory experiments on possible mechanisms for biological control.