Critically ill patients are an important population where infectious diseases are frequently seen and the usage of antimicrobial drugs is very common in these patients. Pathophysiological changes occurring in these patients lead to pharmacokinetic/pharmacodynamic variabilities. There are difficulties in initiating, maintaining, and optimizing antimicrobial therapy in these patients. Undesirable outcomes such as toxicity, mortality, drug resistance, and treatment failures may occur as a result of unnecessary or incorrect drug use. drug monitoring can be used to optimize treatment. In this process, when the plasma concentration of the antimicrobial drug reaches a steady state, blood samples should be taken and drug concentrations should be measured. Antimicrobial drug therapy can be optimized by monitoring the results obtained and the patient's response to treatment. Within the scope of this review, it is aimed to evaluate the benefits provided to patients in this practice by examining the available data on therapeutic drug monitoring of antibiotics, antiviral and antifungal drugs.