This dissertation studies the communication technologies in relaying systems with multiple antennas, especially in the multiple-input multiple-output (MIMO) two-way relaying systems. Both information-theoretic aspects and practical communication strategies are considered and analyzed. For the information-theoretic analysis, an analytical framework for the coverage of MIMO relaying systems based on an outage capacity criterion is proposed. For MIMO two-way relaying systems, different data combining schemes at the relay are compared based on their achievable rates. In addition, optimal time-division (TD) strategies for MIMO two-way decode-and-forward (DF) relaying systems are proposed and analyzed. When the optimal TD strategies are applied, the increase of the achievable rate regions in the system is significant compared to those using the equal TD strategy. For the practical transmission schemes, we propose the self-interference (SI) aided channel estimation and data detection schemes for the broadcast phase of two-way DF relaying systems. Such schemes exploit the SI in two-way DF relaying systems when the superposition coding (SPC) scheme is applied. When the network coding scheme is applied in two-way DF relaying systems, we propose an asymmetric data rate transmission scheme that utilizes the known data bits at the receivers. Such a scheme exploits the a priori known bits at the weak link receiver in the broadcast phase of two-way relaying systems.