today, there is an increasing demand in modern communication systems for smaller antennas, with size significantly less than the usual half wavelength and capable of presenting performance that normally would be achieved by a complicated antenna array. small antenna concept is a challenging aspect that can provide answers and solutions to these problems. the purpose of this thesis is to analyze and to present suggestions in the field of a) functionally small antennas, which definition apply were additional performance has been achieved without increasing the antenna size and b) electrically small antennas which can be physically bounded by a sphere having a radius equal to λ/2π (the small antenna limit). in the first category, a two--branch diversity antenna using a simple printed dual band double--t monopole and operating at 2.4 and 5.2 ghz of ism band is presented in this dissertation. the proposed configuration could be used in laptop computers or wireless routers. a prototype of the proposed antenna has been constructed and tested. the measured results are in good agreement with the computed ones from simulation. τhe diversity criteria are met, while the antenna acquires favorable characteristics such as reduced size and extended bandwidth. in the electrically small antenna category, the process of miniaturization of a rectangular microstrip patch antenna by the insertion of slits parallel to the radiating edges is investigated and a number of exact and approximate expressions, used to calculate the quality factor q for wire antennas, are validated. during the miniaturization process significant decrease in the resonant frequency is observed, depending on the slit’s length and width but at the expense of radiation efficiency and operational bandwidth. the quality factor expressions behavior is checked for simulated and measured data, for the prototypes that have been constructed, and compared to the chu fundamental limit and to wheeler's electrically small antenna definition. the resultant antennas can be characterized as small antennas in accordance to the relevant fundamental limitations. by selecting a medium size reduction, satisfying radiation efficiency is achieved, while the radiation pattern remains unchanged. specifically, for a 50% frequency decrease there is a size reduction equal to 44% and the efficiency remains relatively high around 50%, while the fractional bandwidth decreases to 0.5%. the radiation pattern remains practically unaltered by the miniaturization process. this is important since in the field of electrically small antennas the frequency reduction along with the radiation pattern are the two key characteristics in applications such as rf--tag and nfc technology where a compromise is acceptable as far as it concerns the efficiency and the bandwidth of the antenna.