The use of biologics to treat cancer and autoimmune diseases like lupus, rheumatoid arthritis, and multiple sclerosis has increased as research and technology have advanced over the last several decades. These biologic medicines take advantage of the immune system to either enhance or block its action depending on the condition being treated. It is important for healthcare providers to have a basic understanding of the normal adaptive immune response to understand the mechanism of action and side effects of these drugs. This issue of The Rx Consultant will provide a background on the development, regulation, and naming of biologic drugs, an overview of adaptive immunity, a discussion on drug classes that modulate the immune response, and considerations before and during treatment with immunomodulating biologics.