The advances in modern communication and security monitoring systems have brought new requirements and challenges in terms of compactness and efficient systems. For example, one of the demands for future wireless communication systems is higher data rates. The reconfigurable antennas are a promising solution for the communication systems which need big channel capacity and wide signal coverage. Reconfigurable antenna has many advantages such as changing their frequency, polarization and radiation beam steering at low cost, providing relatively wide band through frequency re-configurability and well interference jamming and multi-path fade. Therefore, reconfigurable antennas have attracted urgent research interest in the past years. This thesis work is about the research of frequency reconfigurable slot antenna for modern communication and security monitoring applications. Frequency re-configurability is commonly implemented through mechanically tuning and electronically tuning including capacitor, PINdiode switch, varactor, RF MEMS and so forth. In this thesis, different frequency tuning techniques for planar antennas are reviewed, e.g. patch and slot antennas due to its ease integration with guided wave structure. A frequency-agile slot antenna based on capacitive loaded parallel strips feed structure is proposed and discussed in the research. Based on the proposed method, frequency tunable slot antenna using capacitive loaded parallel strips feed structure is designed, implemented and measured. By decreasing capacitor’s value for the former one from 1.8 pF to 0.5 pF, its resonant frequency is elevated from 1.2 GHz to the higher frequency regime till 1.77 GHz whilst consistent radiation pattern directions are obtained in the entire tunable range and thus verified the proposed method. Then, a novel single band frequency tunable slot antenna using four varactors is presented. Its operating resonant frequency is tuned from 1.26 GHz to 1.66 GHz with varactors are biased within 0 V to 30 V DC voltages respectively. Meanwhile, consistent radiation pattern directions are obtained in the entire tunable frequency range also. Then, increasing the slot width, a dual-frequency tunable slot antenna is achieved with approximate constant frequency ratio. The first resonant frequency can be tuned from 1.36 GHz to 1.52 GHz while the second resonant frequency can be tuned from 2.12 GHz to 2.63 GHz. The radiation pattern directions approximately remain the same over the whole voltage tuning range for both bands like the previous two single band antennas. The achievable frequency ratio is among range from 1.56 to 1.73 over the whole tunable range. Due to the low gain issue which caused by the varactors and bias circuit, a low noise amplifier (LNA) is designed and integrated to the antenna to improve its gain performance. In order to further improve the active antenna performance, the co-design method is investigated and proposed to adopt.