Oral potentially malignant disorders (OPMD) are a group of mucosal disorders characterised by an increased risk of malignant transformation either in the lesion or elsewhere in the clinically normal-appearing mucosa. The most common OPMD include leukoplakia, erythroplakia, proliferative verrucous leukoplakia, lichen planus and related lesions, and actinic cheilitis. The clinical management of this group of diseases varies according to the exact diagnosis. Therefore, risk assessment plays an important role in informing treatment options. The aim of investigations, mainly biopsy, in OPMD is to assess the presence of carcinoma or the presence and degree of oral epithelial dysplasia. This chapter provides an overview of the current classification of OPMD, including the most recent World Health Organization (WHO) classification of head and neck tumours. Also, it details the histopathological features of OPMD in addition to histological features of cellular and architectural of oral epithelial dysplasia.