  Let $\mathcal{G}$ be an undirected graph with adjacency matrix $A$ and
spectral radius $\rho$. Let $w_k, \phi_k$ and $\phi_k^{(i)}$ be, respectively,
the number walks of length $k$, closed walks of length $k$ and closed walks
starting and ending at vertex $i$ after $k$ steps. In this paper, we propose a
measure-theoretic framework which allows us to relate walks in a graph with its
spectral properties. In particular, we show that $w_k, \phi_k$ and
$\phi_k^{(i)}$ can be interpreted as the moments of three different measures,
all of them supported on the spectrum of $A$. Building on this interpretation,
we leverage results from the classical moment problem to formulate a hierarchy
of new lower and upper bounds on $\rho$, as well as provide alternative proofs
to several well-known bounds in the literature.
