Mycotoxins are toxic secondary metabolites formed by toxigenic fungi present in foods, feeds and their components. Since the presence of the above men¬tioned microorganisms is unavoidable, mold growth is an inevitable consequence of conditions advantageous for fungi development (water content and temperature) found in case of specific food and feed technologies. Mycotoxins identified up to now (several hundred) are molecules of different chemical structures and molecular weight up to 500 Da. An attempt to explain why biosynthesis of so many toxic metabolites – are the consequence of primary metabolism – is necessary for fungi will be discussed. Exposure of human beings and animals to the above compounds causes undesirable effects, with a broad variety of biological effects and – as a consequence – chronic diseases called mycotoxicoses. The most important genera of toxigenic and pathogenic fungi Aspergillus, Fusarium and Penicillium will be characterized, especially concerning conditions of rapid growth and development followed by biosynthesis of a variety of mycotoxins.The above microorganisms are related – during vegetation – to plant oxidative stress induced by weather and environment conditions. The above as well as biosynthesis of the metabolites will be discussed. Rapid development of fungi followed by toxin formation causes a reduction of crop yields, a deterioration of their quality and as a consequence results in significant economic losses, as worldwide approximately 25% of crops are affected by mycotoxins annually. Presented results will confirm that six metabolites (aflatoxins B1, ZEA, DON, fumonisins, T-2 toxin and OTA) in diverse materials (matrices) repre¬sent a real problem throughout the world. Occurrence and concentration levels are variable for different mycotoxins and are closely related to the weather conditions and plant stress responsible for the metabolite formation, which will be highlighted. Levels of safety will be discussed with the emphasis on low doses which cause subclinical losses. Most of the diseases occur after consumption of mycotoxin-contaminated grains or their products, but other routes of exposure also exist. The diagnosis of mycotoxicoses is dependent upon adequate testing for mycotoxins, involving sampling, sample preparation and analysis. Information on mycotoxin contamination of foods, feeds and their components as well as relevant data on new sources of contamination will be given. Methods of decontamination are usually difficult, so prevention of mycotoxin formation is of prime concern. Results on breeding of cereal varieties resistant to pathogenic fungi and biosynthesis of toxic secondary metabolites (mycotoxins) will be presented with an emphasis on the reduction of human and animal health risk. Recent results indicating the importance of mycotoxins in Poland (compared to world data) are planned to be discussed.