This paper presents an overview of normal and sonographic anatomy of the peritoneum and anal canal. It describes first muscles of peritoneum and pelvic floor. Then normal anatomy of the anal canal, followed by endosonographic presentation of anal canal sphincters is presented. Anal endosonography is currently the most commonly used technique for anal canal imaging. The aim of this paper is to present a general overview of muscles of peritoneum, which will be followed by subsequent papers on symptoms of obstetric defects of peritoneum and anal sphincters, and methods of their diagnostics.