Robot capability of listening to several things at once by its own ears, that is,robot audition, is important in improving interaction and symbiosis between humans and robots. The critical issue in robot audition is real-time processing and robustness against noisy environments with high flexibility to support various kinds of robots and hardware configurations. This paper presents two important aspects of robot audition; Missing-Feature-Theory (MFT) approach and active audition. HARK open-source robot audition incorporates MFT approach to recognize speech signals that are localized and separated from a mixture of sound captured by 8- channel microphone array. HARK is ported to four robots, Honda ASIMO, SIG2, Robovie-R2 and HRP-2, with different microphone configurations and recognizes three simultaneous utterances with 1.9 sec latency. In binaural hearing, the most famous problem is a front-back confusion of sound sources. Active binaural robot audition implemented on SIG2 disambiguates the problem well by rotating its head with pitting. This active audition improves the localization for the periphery.