  Let $M_{\mathbb{P}^2}(v)$ be a moduli space of semistable sheaves on
$\mathbb{P}^2$, and let $B^k(v) \subseteq M_{\mathbb{P}^2}(v)$ be the
\textit{Brill-Noether locus} of sheaves $E$ with $h^0(\mathbb{P}^2, E) \geq k$.
In this paper we develop the foundational properties of Brill-Noether loci on
$\mathbb{P}^2$. Set $r = r(E)$ to be the rank and $c_1, c_2$ the Chern classes.
The Brill-Noether loci have natural determinantal scheme structures and
expected dimensions $dim B^k(v) = dim M_{\mathbb{P}^2}(v) - k(k - \chi(E))$.
When $c_1 > 0$, we show that the Brill-Noether locus $B^r(v)$ is nonempty. When
$c_1 = 1$, we show all of the Brill-Noether loci are irreducible and of the
expected dimension. We show that when $\mu = c_1/r > 1/2$ is not an integer and
$c_2 \gg 0$, the Brill-Noether loci are reducible and describe distinct
irreducible components of both expected and unexpected dimension.
