Seborrheic Dermatitis: This condition is a common cause of crusting and itching behind the ear, characterized by an inflammatory response to a fungal infection, often presenting with oily, scaly patches.

Atopic Dermatitis (Eczema): This chronic skin condition can cause itching, crusting, and inflammation behind the ear, especially in individuals with a history of allergies or asthma.
Contact Dermatitis: An allergic reaction or irritation from substances like soaps, shampoos, or jewelry can lead to crusting and itching behind the ear.
Psoriasis: Although less common, psoriasis can manifest behind the ear, causing red, scaly, and itchy patches.
Otitis Externa: An infection of the outer ear canal can cause itching, crusting, and discomfort, often related to bacterial or fungal infections.

Basal Cell Carcinoma or Squamous Cell Carcinoma: Although rare, skin cancers can present with crusting and itching behind the ear, emphasizing the importance of a thorough examination and potential biopsy if suspicious lesions are found.
Lymphoma: Cutaneous T-cell lymphoma (e.g., mycosis fungoides) can cause skin lesions, including behind the ear, which might be itchy and crusty.

Keratosis Obturans: A rare condition where a plug of keratin forms in the ear canal, potentially causing itching and crusting.
Darier's Disease: A genetic disorder that affects the skin, sometimes causing crusting and itching, although it is very rare and typically presents with more widespread skin lesions.
Necrobiotic Xanthogranuloma: A rare skin condition associated with paraproteinemias, which can cause yellowish plaques and nodules, potentially with crusting and itching behind the ear.