2-D and 3-D anal ultrasound are among the most recent and advanced tools available for both the diagnosis and the management of anorectal diseases. They are neither expensive nor harmful for the patients and progressively replaced anal mapping with EMG electrodes for the diagnosis of sphincter's defects and anismus, which represents nearly 50% of the cases of chronic constipation. Anal US may provide the clinician with useful information for both classification, diagnosis and management of anorectal sepsis, anal incontinence and anorectal-perineal chronic pain. Almost any case presented in this Atlas shows both imaging and clinical pictures, thus allowing both the radiologist and the clinician to assess the reliability of the exam and the outcome of the selected treatment.