There are two important trends that seem to be the driving forces in the evolution of communication networks. On the one hand, mobility support becomes a "must" for emerging services and technologies. On the other hand, services tend to have a ubiquitous nature, they are based on intelligent devices being embedded in the ambient environment surroundig us. These new trends result in new research challenges as well. In this paper we first present the specificities of wireless ad-hoc and sensor networks, technologies that were developed to support these new requirements. Then, we highlight the most important and most challenging issues that arise from these specificities.