During Crisis on Infinite Earths, Barry Allen gave his life to save the Earth when destroying an antimatter cannon that was aimed at Earth. In the aftermath of the conflict, Wally took on his fallen mentor’s costume and identity. He became the Flash in Crisis on Infinite Earths #12 (November 28, 1985).
Wally West was less powerful than his predecessor. His differences were highlighted, not just in his crime fighting, but also in his personal life. West won a lottery, bought a large mansion, and began dating beautiful women. Later, the first Reverse-Flash, Eobard Thawne, came from another time period to increase the speed of the character, forcing him to push past a psychological block he had placed on his powers. To prevent himself from truly “replacing” Barry, Wally had subconsciously limited his speed so that he could never become his mentor’s equal, but Thawne’s bragging that he would become the true Flash forced Wally past this block, as he feared Thawne replacing Barry more than he feared himself doing so. After this encounter, he was again Barry Allen’s equal in speed, and eventually became even faster.