Therapeutic drug monitoring is performed to ensure drug levels in the blood are in the therapeutic (not toxic) range, thus imporving efficacy of the drug and patient outcomes. While not all drugs require this, those that do have a recognised desired serum concentration which provides optimal effect. The chapter describes some general rules, including when to take levels and how to calculate ideal and corrected body weight, and runs through some common examples, including gentamicin, phenytoin and aminophylline.