IEEE 802.15.4 is a new enabling technology for low data rate wireless personal networks. This standard was not specifically designed for wireless sensor networks, but it has shown to be a good match with necessary requirements on low data rate, low power consumption and low cost. Unlike the former 802.11 standard, the MAC protocol specified in IEEE 802.15.4 can operate in two different modes: beacon-enabled mode or non-beacon enable mode. In beacon-enabled mode, nodes can exclusively allocate a number of guaranteed time slots, similar to a resource reservation scheme. Hence, the IEEE 802.15.4 MAC protocol have sufficient capabilities for supporting real-time communication. This paper presents the key features of IEEE 802.15.4 which makes it an attractive standard to use for real-time wireless sensor networks. Two real-time protocols extending the IEEE 802.15.4 standard are reviewed. The purpose of this paper is to present the state of the art on real-time support over IEEE 802.15.4 for wireless sensor networks and to discuss the possibilities on improvements on both the standard and the real-time protocols extending the standard.