Antenna is one of the most important components in wireless systems since signal transmission and reception are conducted through the antenna interface. Therefore, the signal quality is highly affected by the properties of the antenna. Traditional antennas integrated in devices such as laptops or cell phones have fixed radiation properties and can not be changed to adapt to different environments. Thus the performance of thefwhole system will be negatively affected since the antenna will not operate in the optimum status in different environments. To solve this problem, reconfigurable antenna, which can dynamically change its operation frequency, radiation pattern, and polarization, has gained a significant interest recently. Recongurable antennas are considered smart antennas, and can maximize the capacity of the wireless system. This dissertation focuses upon the theoretical analysis and design of smart antennas with recongurable radiation properties. The presented multi-functional reconfigurable antennas (MRAs) are aimed to applications in WLAN (wireless local area network) systems. The theoretical analysis of the MRA was rst investigated to validate the design concept, and then applied for practical applications. The multi-functional recongurable antenna array (MRAA), which is a new class of antenna array, is also created as a linear formation (4 1) of MRA, with theoretical analysis and design of the MRAA fully described. This work developed three MRA(A)s for practical implementation in WLAN systems. The rst design is the MRA operating in 802.11 b/g band (2.4-2.5 GHz), with nine beam steering directions in a parasitic layer-based MRA structure. The second is a MRA operating in 802.11ac band (5.17-5.83 GHz) with three beam steering directions in a simplied parasitic layer-based MRA structure. The third is a MRAA extension of the second design. The design process of these MRA(A)s is realized with the joint utilization of electromagnetic (EM) full-wave analysis and multi-objective genetic algorithm. All three MRA(A) designs have been fabricated and measured. The measured and simulated results agree well for both impedance and radiation characteristics. These prototypes can be directly employed in a WLAN system since practical limits have been taken into account with real switches and components implemented. Finally, this dissertation work concludes with plans for future work, which will focus on development of MRA(A)s with dual-frequency operation.