The eyelid skin, especially the lower eyelid, is among the most sunlight-exposed anatomical structures. The eye and the eyelids are one of the most observed parts of the face, and therefore, eyelid tumors are usually diagnosed at an early stage. The eyelid skin is the thinnest in the body and lacks subcutaneous fat, but otherwise contains all other skin structures. In the pretarsal part, the skin and orbicularis oculi muscle are normally firmly attached to the tarsal plate, whereas in the preseptal part they are more loosely attached. The skin epithelium is keratinized stratified squamous epithelium, the origin of all types of benign and malignant epidermal tumors. Melanocytes are spread in the basal layer of the epithelium and may give rise to melanocytic cutaneous lesions. The dermis contains also fibrous tissue, blood and lymphatic vessels, and nerves that can give rise to many types of fibrous tissue tumors, fibrohistiocytic tumors, vascular tumors, and neural tumors.

The eyelids are rich in glandular tissue that may be the origin of various glandular tumors. Eccrine gland tumors may arise from the sweat glands of the eyelid skin as well as from the accessory lacrimal glands of Krause and Wolfring. The gland of Moll can give rise to apocrine tumors. The sebaceous glands of Zeiss and the meibomian gland are the origin of sebaceous gland tumors.

The entire orbital entrance is covered by the orbicularis oculi – a striated muscle that is divided into pretarsal and preseptal zones which are part of the eyelids and are involved in the eyelid movements, and the orbital zone that is located over the external orbital bones.

The tarsi are firm plates composed of dense connective tissue that serve as the skeleton of the eyelids. The upper tarsal plates are much larger than the lower ones. The meibomian glands, large sebaceous glands, are embedded in the connective tissue of the tarsal plates. The superior tarsal muscle (Muller’s muscle) – a smooth muscle, is attached to the upper margin of the tarsus. The upper and lower orbital septum, a thin sheet of fibrous tissue, arises from the periosteum in the orbital rim and fuses with the levator aponeurosis superiorly and the lower margin of the lower tarsus inferiorly. All these histological structures can give rise to rare fibrous, striated, and smooth muscular and glandular tumors. The orbital fat behind the septum and the fat under the orbital part of the orbicularis oculi can be the origin of rare lipomatous tumors.

The posterior eyelid surface is lined by the conjunctiva – a translucent mucous membrane that is composed of epithelium and subepithelial stroma – the substantia propria. The anatomical and histological features of the conjunctiva and the possible tumors that can originate from this tissue are described elsewhere (Chap.​ 13).

The eyelid margin is a flat area on the edge of each margin. The anatomical structures that are seen in the margin from the skin backwards are the eyelashes and their lash follicles, the gray line which consists of the tips of the pretarsal orbicularis muscle – the muscle of Riolan, the meibomian gland orifices, and the mucocutaneous junction just posterior to them.

The venous and lymphatic drainage is important in understanding the routes of possible eyelid tumor metastases. The eyelid has extensive vascularity that comes from two main sources – the internal carotid and external carotid arteries with anastomoses between these two systems. The venous drainage is into the angular vein medially, superficial temporal vein laterally, and the orbital veins, anterior facial vein, and pterygoid plexus posteriorly. The lymphatic drainage of the medial portions of the eyelids is into the submandibular lymph nodes and of the lateral portions into the superficial preauricular nodes and then into the deeper cervical nodes.

The sensory nerve supply to the eyelids is from the fifth cranial nerve, and the motor nerve supply to the striated muscles is from the third and seventh cranial nerves and to the smooth muscles from sympathetic nerves.

Tumors of the eyelid may be classified, like tumors in other organs, according to their tissue or cell of origin and as benign or malignant. In most groups of tumors, unique histological subtypes behave differently in spite of being of the same cell of origin.

The classification of eyelid tumors that appears in this section is based primarily on the second edition of the World Health Organization (WHO) International Histological Classification of Tumors (Table 2.1) [2]. The epithelial tumor classification has been modified and divided into groups according to the tumor cell of origin. Some tumors that are missing from the WHO list have been added from other sources [3–5].

The vast majority of the eyelid tumors, benign and malignant, are of cutaneous origin, mostly epidermal. These tumors are divided into non-melanocytic and melanocytic tumors (Table 2.2). Benign epithelial proliferations, basal cell carcinoma, cystic structures, and melanocytic nevi represent about 85 % of all eyelid tumors [6, 7]. The squamous cell carcinoma and the melanoma are relatively rare [7]. Tumors arising from adnexal structures (Table 2.3), fibrous tissue, fibrohistiocytic and muscular tumors (Table 2.4), and other stromal tumors (Tables 2.5 and 2.6) are less frequent. Lymphoid tumors, hamartomas and choristomas, and inflammatory and infectious lesions that simulate neoplasms are listed in Table 2.7.