A quadrangular embedding of a graph in a surface $Σ$, also known as a quadrangulation of $Σ$, is a cellular embedding in which every face is bounded by a $4$-cycle. A quadrangulation of $Σ$ is minimal if there is no quadrangular embedding of a (simple) graph of smaller order in $Σ$. In this paper we determine $n(Σ)$, the order of a minimal quadrangulation of a surface $Σ$, for all surfaces, both orientable and nonorientable. Letting $S_0$ denote the sphere and $N_2$ the Klein bottle, we prove that $n(S_0)=4, n(N_2)=6$, and $n(Σ)=\lceil (5+\sqrt{25-16χ(Σ)})/2\rceil$ for all other surfaces $Σ$, where $χ(Σ)$ is the Euler characteristic. Our proofs use a `diagonal technique', introduced by Hartsfield in 1994. We explain the general features of this method.