Although anal cancer is a rare disease, its incidence is increasing in men and women worldwide. The most important risk factors are behaviors that predispose individuals to human papillomavirus (HPV) infection or immunosuppression. Anal cancer is generally preceded by high-grade anal intraepithelial neoplasia (HGAIN), which is most prevalent in human immunodeficiency virus (HIV)-positive men who have sex with men. There is a general consensus that high-risk individuals may benefit from screening. Meta-analysis suggests that 80% of anal cancers could be avoided by vaccination against HPV 16/18. Nearly half of all patients with anal cancer present with rectal bleeding. Pain or sensation of a rectal mass is experienced in 30% of patients, whereas 20% have no tumor-specific symptoms. According to the Surveillance Epidemiology and End Results (SEER) database, 50% of patients with anal cancer have disease localized to the anus, 29% have regional lymph node involvement or direct spread beyond the primary, and 12% have metastatic disease, while 9% have an unknown stage. Clinical staging of anal carcinoma requires a digital rectal exam and a computed tomography scan of the chest, abdomen, and pelvis. Suspicious inguinal lymph nodes should be subject to pathologic confirmation by fine-needle aspiration. The 5-year relative survival rates are 80.1% for localized anal cancer, 60.7% for regional disease, and 29.4% for metastatic disease. Part 2 of this two-part review will address the treatment of anal cancer, highlighting studies of chemoradiation.