If

$X$ is a separable infinite-dimensional Banach space, a

${C}_{0}$-semigroup

${\left({T}_{t}\right)}_{t\ge 0}$ of bounded linear operators on

$X$ is said to be

*hypercyclic* if there exists a vector

$x\in X$ such that

$\{{T}_{t}x\mid t\ge 0\}$ is dense in

$X$, and

*frequently hypercyclic* if there exists a vector

$x\in X$ such that for any non-empty open subset

$U$ of

$X$, the set

$\{t\ge 0\mid {T}_{t}x\in U\}$ has positive lower density. In this paper, the authors prove a version for

${C}_{0}$-semigroups of the so-called Frequent Hypercyclicity Criterion. Applications are given to semigroups generated by Orstein-Uhlenbeck operators, in particular to translation semigroups on weighted spaces of

${L}^{p}$-functions or continuous functions which, when multiplied by the weight, vanish at infinity.