  Given a graph $G$, an {\em obstacle representation} of $G$ is a set of points
in the plane representing the vertices of $G$, together with a set of connected
obstacles such that two vertices of $G$ are joined by an edge if and only if
the corresponding points can be connected by a segment which avoids all
obstacles. The {\em obstacle number} of $G$ is the minimum number of obstacles
in an obstacle representation of $G$. It is shown that there are graphs on $n$
vertices with obstacle number at least $\Omega({n}/{\log n})$.
