This column first reviews 2 key equations that are central to clinical pharmacology. Clinicians can use the first equation to predict the effect of a specific dose of a specific drug in specific circumstances on the basis of 3 variables: (1) the drug's pharmacodynamics, (2) the drug's pharmacokinetics, and (3) biological variance in the individual patient. Clinicians can use the second equation to determine the concentration of a drug that a patient will achieve on a given dose depending on the patient's ability to clear the drug from the body. These 2 equations allow prescribers to predict whether the dose of a drug a patient is receiving is likely to achieve the desired clinical response (not so low that it is clinically ineffective or so high that it causes adverse effects that interfere with the patient's ability to tolerate or benefit from the treatment). The author then describes 2 tools clinicians can use to determine a patient's ability to clear a drug from the body, and thus calculate the concentration of the drug using Equation 2. These tools are: (1) estimation of creatinine clearance and (2) therapeutic drug monitoring.