There are four types of anastomoses between themedian and ulnar nerves in the upper limbs. It consists of crossings of axons that produce changes in the innervation of the upper limbs, mainly in the intrinsic muscles of the hand. The forearm has two anatomical changes – Martin-Gruber: branch originating close to the median nerve joining distally to the ulnar nerve; and Marinacci: branch originating close to the ulnar nerve and distally joining the median nerve. The hand also has two types of anastomoses, which are more common, and sometimes considered a normal anatomical pattern – Berrettini: Connection between the common digital nerves of the ulnar and median nerves; and Riche-Cannieu: anastomosis between the recurrent branch of the median nerve and the deep branch of the ulnar nerve. Due to these connection patterns, musculoskeletal disorders and neuropathies can be misinterpreted, and nerve injuries during surgery may occur, without the knowledge of these anastomoses. Therefore, knowledge of them is essential for the clinical practice. The purpose of the present review is to provide important information about each type of anastomosis of the median and ulnar nerves in the forearm and hand.