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The liver, particularly its smooth endoplasmic reticulum and cytosol, is the primary site of xenobiotic biotransformation; the intestine, lungs, kidneys and skin also contribute. The overall aim is to make lipophilic compounds more water-soluble for excretion, commonly through urine or bile. Metabolism is organized into three phases: Phase I modifies or exposes functional groups, Phase II attaches hydrophilic groups, and Phase III transports compounds across membranes for efflux. These processes can be altered by enzyme induction or inhibition, genetic variation, and factors such as age, liver disease or malnutrition. Some Phase I reactions generate reactive intermediates that may contribute to cellular injury, making metabolism clinically relevant to both drug response and toxicity.