Inflammatory and infectious processes in the neck and mediastinum can be severe, particularly descending necrotizing mediastinitis (DNM), where an infection originating in the neck spreads to the mediastinum. This can lead to acute mediastinal syndrome, a critical condition marked by symptoms indicating acute mediastinal impairment, with a mortality rate reaching 40% if untreated. This chapter highlights the role of emergency CT in the early identification of acute necrotizing mediastinitis and its causes. Acute mediastinitis can arise from several sources, with the most common being esophageal tears, thoracic surgeries, and cervical infections leading to DNM. The ITMIG 2017 classification of mediastinal compartments aids in understanding the spread of infections from neck spaces. The distinction between focal and diffuse DNM guides the decision for conservative or interventional treatment. This chapter details the primary causes of DNM, including muscle, odontogenic, and tonsillar abscesses, as well as foreign body mediastinitis and sternoclavicular septic arthritis. The discussion underscores the pivotal role of CT in early diagnosis, determining the etiology, and selecting appropriate treatment.