Pharmacology is the study of how medications interact with the human body. Pharmacokinetics and pharmacodynamics are the two major branches of pharmacology. Pharmacokinetics (PK) is described as the movement of medications through the body, whereas pharmacodynamics (PD) is defined as the biological response of the body to pharmaceuticals. PK characterizes absorption, distribution, bioavailability, metabolism, and excretion as a function of time to describe a drug's exposure. Drug response is described in terms of biochemical or molecular interactions in PD. Every drug's development and approval hinges on an understanding of the exposure-response relationship (PK/PD). Much of what is on a drug package insert is based on PK and PD data. The development of a drug can be accelerated by strategic planning of the overall program and intelligent pharmacokinetic study design.