US is a powerful tool for characterization of superficial lymph nodes. Unlike other imaging techniques like CT and MRI, which mainly depend on size for differential diagnosis, US can evaluate important parameters such as shape, margins, internal structure and abnormal vascularization. It has the advantage of showing early subtle findings of neoplastic involvement like asymmetric thickening and focal lobulations in the cortex, as well as late findings such as diffuse cortical thickening and abscence of hilum. It is possible to evaluate the heterogeneous inner structure due to necrosis, unsharp or irregular borders due to extracapsular spread and abnormal vascularization with multiple subcapsular feeding vessels due to tumor angiogenesis. The advances in US technology like high-resolution transducers, power Doppler function and contrast agents have all contributed to the well established role of US in the evaluation of lymph nodes. This article mainly focuses on the sonographic criteria for differential diagnosis of normal, reactive, inflammatous and neoplastic lymph nodes as well as examination techniques in the neck, axilla, internal mammary and inguinal regions and indications in various clinical settings. (c) 2006 Elsevier Ireland Ltd. All rights reserved.