Hard bumps on the skin can be caused by dermatofibromas, sebaceous cysts, and folliculitis. 1. Dermatofibroma: Dermatofibroma, also known as nodular subepidermal fibrosis, is a reactive proliferation of fibroblasts triggered by small skin injuries. Typical lesions are slow-growing round or ovoid firm nodules with smooth or rough surface, often single. Generally no treatment is needed, but surgical excision and histopathologic examination if necessary. 2. Epidermoid cysts: Epidermoid cysts are one of the most common skin cysts, which can occur in any part of the skin. The lesions are well-demarcated nodules, and the contents of the cyst can be squeezed out with an unpleasant odor. Surgical excision can cure epidermoid cysts is a common treatment. 3. Folliculitis: Folliculitis is a purulent inflammation confined to the mouth of the hair follicle, and is most common on the head, face, neck, buttocks and vulva. High temperature, excessive sweating, scratching, poor hygiene habits, etc. are common triggering factors. The lesions begin as red follicular papules, with pustules appearing in the center within a few days, and yellow scabs forming after the pustules dry up or break down. It can be treated with topical antibacterial ointment such as mupirocin. Hard bumps on the skin may also be caused by other reasons, and should be consulted as soon as possible, after a clear diagnosis, under the guidance of the doctor for treatment.