Real-time wireless sensor networks become more and more important in emerging new applications as message delivery timeliness is highly concerned. However, supporting real-time QoS in sensor networks has faced many challenges due to their wireless nature, limited resource, dynamic network topology, and the demand of distributed architecture. There are tradeoffs between different application requirements including energy efficiency and delay performance. This paper studies the state of the art of current real-time solutions including MAC protocols, routing protocols, data aggregation strategies, and cross-layer designs. Some research challenges and design favors are also identified. The discussion may offer a reference for future investigations.