During the last decades, the stability problems of several functional equations have been widely studied. In this thesis we first study the general solution and the generalized Hyers-Ulam-Rassias stability of the quartic functional equation ƒ(3x+y) + ƒ(x+3y) = 64 ƒ(x) + 64 ƒ(y) + 24 ƒ (x+y)-6 ƒ(x-y) and then extend the idea to investigate the general solution and the generalized Hyers-Ulam-Rassias stability of the pentic functional equation ƒ(x+5y) - 5ƒ(x+4y)+10ƒ(x+3y) - 10ƒ(x+2y) + 5ƒ(x+y) - ƒ(x) = 120ƒ(y).