Recently, several research programs have been started to develop new electronic antennas by a new approach called Electronically Reconfigurable Antenna (ERA). Electronically reconfigurable antenna is new promising area of antenna designs that alters the key antenna element properties (radiation pattern, polarization, radar cross section, tuning and input impedance) in a non-mechanical but controlled manner.This dissertation focuses on the development of new designs of electronically reconfigurable antennas. The research covers a wide range of interesting issues about new designs of reconfigurable antenna, from theoretical analysis and design to measurement results, from basic concepts to various applications. The main contributions or creative work of the dissertation are as follows.Design principle of reconfigurable single-feed circular microstrip antenna for dual polarization and dual frequency operation is presented. The effects of different parameters on the antenna performance are analyzed.A novel design concept of reconfigurable single-feed microstrip antenna for polarization diversity has been proposed. It is demonstrated that by incorporating a cross-slot of equal slot arm lengths in the patch antenna and utilizing two PIN diodes to switch the slots ON or OFF, Right Hand Circular Polarization (RHCP) or Left Hand Circular Polarization (LHCP) can be achieved using the same feeding probe. The new design concept is successfully applied to design a circular microstrip patch antenna for polarization diversity. The antenna design and experimental results are presented and discussed.In the previous design of the reconfigurable antenna PIN diode or Radio Frequency Micro Electrical Mechanical Systems (RF MEMS) switches have been proposed to realize the reconfigurabiltiy of the antenna. In this work also, lumped elements have been proposed to control the reconfigurability of antennas. A reconfigurable antenna for polarization diversity was succesfully designed by implementing a capacitor in the circular patch.A new design of reconfigurable single-feed circular patch microstrip antenna for dual-band circular polarization application have been also proposed. The dual-band functionality is realized through incorporating cross-slots of equal slot length in the circular patch and utilizing two PIN diodes to switch the slots on or off. A pairs of tuning stubs are used to tune the circular polarization performance. Prototype antennas were made and tested. A good results were obtained.A new reconfigurable muti-band antenna has been designed. The antenna consists of 4x4 array of microstrip patch antennas. The new antenna architecture can operates at multiple widely separated frequency bands, S/X/Ku-bands. Single broadband or dual-band behavior was obtained. The computer simulations, design procedure and measurement results for the reconfigurable multi-band antenna are given.The design of a beam steering electronically reconfigurable antenna is introduced. Applications using eight passive monopole elements and one folded active element operating at 2.1 GHz are presented. In order to get beam steering, the switches control the states of the parasitic monopoles to change their impedance and current distribution. The methods used to analyze the direction finding are assessed. The electronically reconfigurable antenna can steer its beam 360 with its parameter consistency such as radiation pattern, input impedance, etc, without many RF amplifiers, many phase shifter and complex feed structures.A new type of PBG structure with circular symmetry has been proposed. The reconfigurable microstrip antenna with circular polarization has further been investigated integrating with the new planar PBG. A significant improvement has been observed in the impedance bandwidth and the axial ratio bandwidth.