Owing to numerous potential applications, wireless sensor networks have been the focus of a lot of research efforts lately. In this note we study one fundamental issue in such networks, namely the coverage problem, in which we would like to determine whether a region of interest is sufficiently covered by a given set of sensors. This problem is motivated by monitoring applications in sensor networks, as well as robustness concerns and protocol requirements. We show that the coverage problem and some of its variants can be treated in a unified manner using suitable generalizations of the Voronoi diagram. As a result, we are able to give algorithms that have better runtimes than those proposed in previous works (see, e.g., [5, 6]). Our approach also yields efficient algorithms for coverage problems where the sensing region of a sensor is