In evaluating veterinary patients, lymphadenopathy is a common and significant clinical finding. Recognizing that one or more lymph nodes are altered in size or texture or noting the presence of peripheral limb edema may provide important diagnostic clues when assessed in conjunction with historical facts and other clinical findings. Part I of this two-part presentation describes the pertinent features of lymph node development, architecture, and function. Knowledge of the anatomy of normal lymphoid structures and lymphatic drainage patterns is emphasized, as is the importance of the lymph node as a branch of the immune system. Part 11 will discuss the diagnostic evaluation of canine and feline patients with lymphadenopathy.