Secondary metabolites are organic compounds produced by microorganisms, plants, or animals that are not directly involved in growth, development, or reproduction. Unlike primary metabolites, they often have specialized roles such as defense, signaling, or competition. These compounds include antibiotics, pigments, toxins, and alkaloids. Microorganisms like fungi and actinomycetes are known for producing valuable secondary metabolites. They are widely used in medicine, agriculture, and industry. Production typically occurs during the stationary phase of microbial growth. Secondary metabolites often have complex structures and diverse biological activities. Their discovery has led to major advances in drug development and biotechnology.