||The ever diminishing size and increasing transmission speed of modern mobile communication systems has placed significant challenges on the further development of antenna technology. The emergence of the System−on−Chip (SoC) concept has boosted research on the integration of antennas into Silicon Radio Frequency Integrated Circuit (RFIC) chips. This could help to further diminish the size and reduce the cost of mobile devices. However, the integration of antennas on silicon chips is not an easy task because of the radiation efficiency and antenna size issues. On the other hand, the recent commercial application of the Multiple Input Multiple Output (MIMO) technology has also led to a multi−fold increase in the transmission speed of Wireless communication systems. These gains will not be possible without the advancement of MIMO antenna technology. Hence it is believed that on−chip and MIMO antennas are two of the most important topics in antenna research for wireless communications. This dissertation considers both the integration of antennas on silicon chip and novel MIMO antenna designs for future wireless communications. The research on on−chip antennas is mainly focused on the enhancement of radiation efficiency and novel compact designs. Compact on−chip antennas with reasonably high radiation efficiency and gain have been designed and fabricated. A detailed study on a fully reconfigurable on−silicon antenna using integrated Micro-Electro−Mechanical System (MEMS) switches is also addressed. For MIMO antenna research, the work is concentrated on designing MIMO antennas with low mutual coupling. Novel orthogonally polarized antennas have been designed to exploit both the spatial and polarization diversity for channel capacity enhancement. The idea is also further extended to the design of a vector sensor antenna which is able to sense and detect distinguishable electric and magnetic fields on the three spatial axes. A novel 48−port MIMO cube antenna constructed using the proposed vector sensor antenna has also been suggested. In addition, a compact 36 mm × 36 mm 4−port MIMO antenna has also been proposed for application in mobile handsets.