An L(2,1)-labeling of a graph G is a function f : V (G) → N∪{0} such that for all u, v in V(G), we have |f(u) − f(v)| is not less than 2 if d(u,v) = 1, and |f(u) − f(v)| is not less than 1 if d(u,v) = 2. The L(2,1)-labeling number λ(G) of G is the smallest number k such that G has an L(2, 1)-labeling with max{f(v) : v in V (G)} = k. In this thesis, we review some proofs for the upper bounds of λ(G), and give an alternative proof for λ(G) is less than or equal to Δ^2+Δ−2.