We study the geometry and topology of properly embedded
minimal surfaces in $M\times\mathbb{R}$, where $M$ is a
Riemannian surface. When $M$ is a round sphere, we give
examples of all genus and we prove such minimal surfaces have
exactly two ends or equal $M\times\{t\}$, for some real
$t$. When $M$ has non-negative curvature, we study the
conformal type of minimal surfaces in $M\times\mathbb{R}$, and
we prove half-space theorems. When $M$ is the hyperbolic
plane, we obtain a Jenkins-Serrin type theorem.