There are three distinct stages of life with unique pharmacokinetic (PK) and pharmacodynamic (PD) characteristics that dictate particular considerations when it comes to the therapeutics of disease management: pediatrics, pregnancy and lactation, and geriatrics. For each stage, unique elements must be factored into the decision-making process when determining appropriate pharmacologic therapy. The primary PK parameters, clearance (CL) and volume of distribution (V_D) may change over the lifespan for a given drug. However, the fundamental equations used for calculating these parameters from patient-specific data are not age dependent. When evaluating PK/PD changes over the lifespan, it is critical to understand the underlying physiological changes that contribute to alterations in clearance and volume. This chapter discusses the PK and PD changes seen in pediatric patients, pregnant and lactating women, and the geriatric population. It also discusses the therapeutic approaches and caveats to treating each stage.