  Let $[n]:=\lbrace 1,2,\ldots,n \rbrace$, and $M$ be a set of positive
integers. Denote the family of all subsets of $[n]$ with sizes in $M$ by
$\binom{\left[n\right]}{M}$. The non-empty families
$\mathcal{A}\subseteq\binom{\left[n\right]}{R}$ and $\mathcal{B}\subseteq
\binom{\left[n\right]}{S}$ are said to be cross $t$-intersecting if $|A\cap
B|\geq t$ for all $A\in \mathcal{A}$ and $B\in \mathcal{B}$. In this paper, we
determine the maximum sum of sizes of non-empty cross $t$-intersecting
families, and characterize the extremal families. Similar result for finite
vector spaces is also proved.
