Anal intraepitelial neoplasia (AIN) constitutes a major health problem in certain risk groups, such as patients with immunosuppression of varied origin, males who have sexual relations with other males, and females with a previous history of vaginal or cervical abnormalities in cytology. Its relationship with the human papillomavirus (HPV) infection has been well documented; however, many of the factors involved in the progression and regression of the viral infection to dysplasia and anal carcinoma are unknown. AIN can be diagnosed through cytology of the anal canal or biopsy guided by high-resolution anoscopy. However, the need for these techniques in high-risk groups remains controversial. Treatment depends on the risk factors and given the high morbidity and high recurrence rates the utility of the different local treatments is still a subject of debate. Surgical biopsy is justified only in the case of progression suggesting lesions. The role of the vaccination in high-risk patients as primary prevention has been debated by different groups. However, there is no general consensus on its use or on the need for screening this population.