The main goal of this paper is to characterize limit key polynomials for a valuation $��$ on $K[x]$. We consider the set $��_��$ of key polynomials for $��$ of degree $��$. We set $p$ be the exponent characteristic of $��$. Our first main result (Theorem 1.1) is that if $Q_��$ is a limit key polynomial for $��_��$, then the degree of $Q_��$ is $p^r��$ for some $r\in\mathbb N$. Moreover, in Theorem 1.2, we show that there exist $Q\in��_��$ and $Q_��$ a limit key polynomial for $��_��$, such that the $Q$-expansion of $Q_��$ only has terms which are powers of $p$.