Wireless Body Area Sensor Networks (WBANs) are becoming more and more popular and have shown great potential in real-time monitoring of the human body. With the promise of cost effective, unobtrusive, and unsupervised continuous monitoring, WBANs have attracted a wide range of monitoring applications such as healthcare, sport activity and rehabilitation systems. However, in using the advantage of WBANs, a number of challenging issues should be resolved. Besides open issues in WBANs such as standardization, energy efficiency and Quality of Service (QoS), security and privacy issues are one of the major concerns. Since these wearable systems control life-critical data, they must be secure. Nevertheless, addressing security in these systems faces some difficulties. WBANs inherit most of the well known security challenges from Wireless Sensor Networks (WSN). However, typical characteristics of WBANs, such as severe resource constraints and harsh environmental conditions, pose additional unique challenges for security and privacy support. In this chapter, we will survey major security and privacy issues and potential attacks in WBANs. In addition, we will explain an unsolved quality of service problem which has great potential to pose a serious security issues in WBANs, and then we discuss a potential future direction.