Sebaceous cysts, commonly known as “pimples,” are cysts that form when sebaceous glands accumulate in the gland due to obstruction of the sebaceous ducts. This is one of the most common benign skin tumors, and many people have had the experience of having a cyst, especially young people in their prime growth spurt. Sebaceous cysts are most common on the scalp and face, followed by the trunk. Because of their varying depths and contents, they vary in size from small ones the size of a grain of rice to large ones the size of an egg. They are often diagnosed as lipomas, fibroids, etc. Sebaceous cysts grow very slowly. Sebaceous cysts are mostly solitary, occasionally multiple, round in shape, moderately hard or elastic, raised above the skin surface, usually within 1 mm, with a smooth surface that feels attached to the surface when pushed but not adherent to the base, and without a sense of fluctuation. The skin color may be normal or pale blue, and the surface skin may shine when enlargement is too fast. Sometimes there is an opening on the surface of the skin from which white tofu-like contents can be squeezed out. This opening is where the opening to the sebaceous glands on the skin surface is located, and the depression of the opening is due to insufficient length of the duct. Sebaceous cysts are often complicated by infection, causing the cysts to rupture and temporarily subside, but they can form scarring and are prone to recurrence. The chance of sebaceous cysts becoming cancerous is extremely rare. Ultrasound shows a cystic mass under the skin with homogeneous internal echogenicity, and in larger ones, the internal echogenicity is seen as tiny dots, which move when squeezed, with no internal blood flow signal. The difference between epidermal cysts and sebaceous cysts is that epidermal cysts contain only skin tissue, i.e., squamous epithelial cells and lipids (triglycerides and cholesterol, etc.); whereas sebaceous cysts also contain skin accessory tissue, such as sebaceous glands, hair follicles, hair, and other structures. Epidermolysis bullosa (epidermoid cyst) comes from the ectoderm and is also known as keratin cyst, which is a benign congenital mass. Occasionally, it can be caused by trauma and friction. The wall of the cyst is normal epidermis, containing keratin and sometimes cholesterol crystals, which can be solitary or multiple. It starts as a small nodule-like mass and gradually increases in size, usually adhering to the epidermis but not to the subcutaneous tissue, and is movable, painless and non-ruptured. They appear as hard, small round swellings the size of a pea and are not painful. Epidermoid cysts that occur within the meninges often accumulate with layered epidermal keratinized scales that have a distinctive silvery white color called cholesteatoma. The cysts can become infected secondary to infection, and after infection, the cysts can adhere to the surrounding tissues, and very rarely can become malignant. Dermoid cysts are congenital benign masses of ectodermal origin. They can occur in many parts of the body such as subcutaneous, soft tissue, sacrococcygeal, ovarian, mediastinal and spinal canal. The facial area is particularly affected by the periocular area and the root of the nose outside the eyebrows. It is also common in the occipital region. The wall of the cyst is made up of connective tissue and lined with the same squamous epithelium as the skin, with hair, sebaceous glands, sweat glands and other tissues inside the wall. The cystic cavity contains a yellowish oily fluid including sebum, shed epithelium and hairs. The swelling is mostly solitary, occurs deep in the subcutaneous soft tissue, is round, and is immobile due to adhesion to the basal fascia.