  In this paper we define and explore the analytic spread $\ell(\mathcal I)$ of
a filtration in a local ring. We show that, especially for divisorial and
symbolic filtrations, some basic properties of the analytic spread of an ideal
extend to filtrations, even when the filtration is non Noetherian. We also
illustrate some significant differences between the analytic spread of a
filtration and the analytic spread of an ideal with examples.
  In the case of an ideal $I$, we have the classical bounds
$\mbox{ht}(I)\le\ell(I)\le \dim R$. The upper bound $\ell(\mathcal I)\le \dim
R$ is true for filtrations $\mathcal I$, but the lower bound is not true for
all filtrations. We show that for the filtration $\mathcal I$ of symbolic
powers of a height two prime ideal $\mathfrak p$ in a regular local ring of
dimension three (a space curve singularity), so that $\mbox{ht}(\mathcal I) =2$
and $\dim R=3$, we have that $0\le \ell(\mathcal I)\le 2$ and all values of 0,1
and 2 can occur. In the cases of analytic spread 0 and 1 the symbolic algebra
is necessarily non-Noetherian. The symbolic algebra is non-Noetherian if and
only if $\ell(\mathfrak p^{(n)})=3$ for all symbolic powers of $\mathfrak p$
and if and only if $\ell(\mathcal I_a)=3$ for all truncations $\mathcal I_a$ of
$\mathcal I$.
