The presence of unwanted bacteria in food, water, or on surfaces, the central concern of food microbiology. Contamination may come from raw ingredients, dirty equipment, unwashed hands, or the environment, and it divides into spoilage organisms that ruin quality and pathogens that cause illness. Whether contamination becomes dangerous depends on the species, the dose, and the food’s ability to support growth: a few Salmonella cells on dry crackers pose little risk, while the same cells in warm chicken can multiply to infectious levels within hours. Control rests on the familiar pillars of clean, separate, cook, and chill, plus validated sanitation in processing plants. Testing for indicator organisms verifies that these controls work, since directly testing for every pathogen is impractical.