Therapeutic drug monitoring is a concept that has its roots in the 1960s, and has evolved today to become a crucial tool in clinical pharmacology. Therapeutic drug monitoring is now an aspect of “personalized therapeutics”. This concept uses body fluids, most commonly plasma, to measure the level of a particular drug in a person’s body. This measured value then guides the clinician in making informed decisions for the betterment of individualized patient care. Therapeutic drug monitoring is particularly useful for drugs that have narrow therapeutic indices, no active metabolites, or no clinical signs of toxicity or effectiveness. This process involves a multidisciplinary team that needs to work together for screening, measuring, analyzing, and interpreting the results. Common drug classes that benefit from therapeutic drug monitoring include anti-epileptics, antipsychotics, antidepressants, etc. It is also important to consider the key limitations of this tool when interpreting the results. This chapter deals with the essentials of therapeutic drug monitoring in today’s practice.