As a novel information acquiring and processing technology, compared to other traditional sensor networks, multimedia sensor networks pay more attention to the information-intensive data (e.g. audio, video, image). Potential applications of multimedia sensor networks span a wide spectrum from military to industrial, from commercial to environmental monitoring. This paper introduces the concept and characteristics of multimedia sensor networks, and discusses the technical challenges in this field. In particular, this paper summarizes the current research progresses. Finally, the open research problems are also pointed out. Multimedia sensor networks discussed in this paper is a novel conceptual system; and many issues need to be solved. The research on this topic is with great theoretical and practical value.