While mobile sensing applications are booming, the sensor man- agement mechanisms in current smartphone operating systems are left behind - they are incomprehensive and coarse-grained, expos- ing a huge attack surface for malicious or aggressive third party apps to steal user's private information through mobile sensors. In this paper, we propose a privacy-aware sensor management framework, called SemaDroid, which extends the existing sensor management framework on Android to provide comprehensive and fine-grained access control over onboard sensors. SemaDroid al- lows the user to monitor the sensor usage of installed apps, and to control the disclosure of sensing information while not affect- ing the app's usability. Furthermore, SemaDroid supports context- aware and quality-of-sensing based access control policies. The en- forcement and update of the policies are in real-time. Detailed de- sign and implementation of SemaDroid on Android are presented to show that SemaDroid works compatible with the existing An- droid security framework. Demonstrations are also given to show the capability of SemaDroid on sensor management and on defeat- ing emerging sensor-based attacks. Finally, we show the high effi- ciency and security of SemaDroid.