Mycotoxins are commonly found in food products and beverages and the exposition to these compounds can cause pathologies known as mycotoxicoses. The main mycotoxin-producing species are included in four genera: Aspergillus, Penicillium, Fusarium and Alternaria. This article is focused on the main mycotoxins produced by these genera and their acute and chronic effects, main producers, structure and occurrence are discussed. Moreover, due to their toxic properties, the maximum levels of many mycotoxins and legislated in different countries; therefore, the current regulation in each case is also indicated.