Humans have now been growing and storing enough food for a long enough time that some rapidly evolving organisms, such as fungi, are moving into niches created by the exploitation of certain plants as food. Food is expected to be nutritious. The most important of the physicochemical conditions that affects fungal growth is related to the biological state of the food. Living foods, particularly fresh fruits, vegetables, and also grains and nuts before harvest, possess powerful defense mechanisms against microbial invasion. When the specific microorganisms overcome defense mechanisms, the spoilage of a living food starts. Other factors to consider are water activity, hydrogen ion concentration, temperature, gas tension, consistency, nutrient status, specific solute effect, and preservation. The consequences of mold contamination of foods are diverse: unsightly appearance, chemical (removal or change of most of the constituents) and nutritional value changes, modification of organoleptic quality, difficulties in preservation, occupational hazards (mycoses, allergies), and toxicoses (mycotoxicoses). It is possible to recognize a succession of three distinct mycoflora during the storage of cereals, but they can also be mixed: 1. Field fungi growing and established before harvesting (Alternaria, Fusarium, Helminthosporium, Cladosporium). 2. Storage fungi taking over and dominanting in the silo (Aspergillus and Penicillium). 3. Advanced decay fungi (Papulospora, Sordaria, Fusarium graminearum, and members of the order Mucorales).