Sweet's syndrome, also referred to as acute febrile neutrophilic dermatosis, is characterized by a constellation of symptoms and findings: fever, neutrophilia, erythematous and tender skin lesions that typically show an upper dermal infiltrate of mature neutrophils, and prompt improvement of both symptoms and lesions after the initiation of treatment with systemic corticosteroids. Hundreds of patients with this dermatosis have been reported. The manifestations of Sweet's syndrome in these individuals have not only confirmed those originally described by Dr Robert Douglas Sweet in 1964, but have also introduced new features that have expanded the clinical and pathologic concepts of this condition. The history, clinical characteristics, laboratory findings, associated diseases, pathology, and treatment options of Sweet's syndrome are reviewed. The evolving and new concepts of this dermatosis that are discussed include: (i) Sweet's syndrome occurring in the clinical setting of a disease-related malignancy, or medication, or both; (ii) detection of additional sites of extracutaneous Sweet's syndrome manifestations; (iii) discovery of additional Sweet's syndrome-associated diseases; (iv) variability of the composition and/or location of the cutaneous inflammatory infiltrate in Sweet's syndrome lesions; and (v) additional efficacious treatments for Sweet's syndrome.