Panniculitis is a collective term for a group of inflammatory diseases involving subcutaneous fat. The panniculitides are subdivided histologically based on the location of the predominant amount of inflammation: in the intralobular septa or within the fat lobule itself. A mixture of patterns may also be seen (septal and lobular). Most share a common presentation of tender dermal or subcutaneous nodules, papules, or plaques. They all may be mistaken for cellulitis.

Erythema nodosum (EN) is by far the most common type of panniculitis and one of few mainly involving the septa (septal panniculitis). Other rarer types of panniculitis are generally classified as more lobular in nature and include infectious (necrotizing granulomatous) panniculitis, a panniculitis associated with systemic lupus erythematosus (lupus profundus), Weber-Christian disease, nodular vasculitis (erythema induratum), pancreatic panniculitis, cytophagic histiocytic panniculitis, and α₁-antitrypsin deficiency panniculitis. There are also physical forms of panniculitis (cold panniculitis, factitial panniculitis). Furthermore, other diseases may be associated with a secondary panniculitis (necrobiosis lipoidica, polyarteritis nodosa, and some vasculitides).

EN is an inflammatory reaction pattern, sometimes precipitated by identifiable endogenous or exogenous stimuli. Streptococcal infections are the most common etiologic factor in children, while sarcoidosis, inflammatory bowel disease, and drugs are more commonly implicated in adults. Most cases are idiopathic. The eruption persists for 3–6 weeks and spontaneously regresses without scarring or atrophy. Fever with generalized aching sometimes accompanies the onset of skin lesions. EN can occur at any age, but most cases occur between the ages of 20 and 45. It is more common in women.

Panniculitis can be distinguished from cellulitis because it most often occurs bilaterally and lesions are often multifocal.