1. Gabrielle Geddes, MD* 2. Mark M. Butterly, MD*,† 3. Sonali Mehta Patel, MD*,† 4. Silvio Marra, MD* 1. *Advocate Hope Children’s Hospital, Oak Lawn, IL. 2. †Rosalind Franklin University of Medicine and Science, North Chicago, IL. * CRP: : C-reactive protein CT: : computed tomography ESR: : erythrocyte sedimentation rate FNA: : fine needle aspiration MRI: : magnetic resonance imaging A variety of lesions, some malignant, can manifest as masses in the neck, presenting real diagnostic challenges. Clinicians must understand the embryologic development of the head and neck and be familiar with the most recent diagnostic techniques. After completing this article, readers should be able to: 1. Know the differential diagnosis of the pediatric neck mass. 2. Understand how to conduct an evaluation of the pediatric neck mass. The broad differential diagnosis of pediatric neck masses typically fits into two main categories: congenital and acquired. Appreciation of the head and neck formation can simplify and greatly assist in identification of the underlying condition. We first review the embryonic formation as it relates to head and neck pathology, followed by key points of history and physical examination. The differential diagnostic possibilities are separated into categories of congenital and acquired neck masses. Finally, diagnostic clues and evaluation options are discussed. The head and neck are derived from the branchial apparatus, an anatomically distinct structure that undergoes a complex formative process involving generation and resorption. Appreciating some aspects of the degeneration process helps explain the relationship between anatomic structures as well as the actual formation process. By the fourth week of gestation, the individual branchial arches initially become visible. Each arch has its own group of uniquely designated tissue types. For simplification, arch structures will be referred to as a numbered arch without differentiation of clefts, pouches, or other structures. The six paired branchial arches forming the branchial apparatus have ectodermal, mesodermal, and endodermal components, each with its own unique designations and components as listed in Table 1. Branchial arches are numbered …