This thesis mainly discusses reconfigurable slot based microwave filters and antennas. In the former part, microwave band pass filters (BPF) based on the slot structures are proposed. Based on the single-band BPF with proposed simplest slot structure, dualband, tri-band and quad-band BPFs are proposed and investigated. Parametric studies of these BPFs are carried out for better understanding on the effect of each sections of slot structure and for achieving desired frequency response. Besides, equivalent circuits for each BPF are also proposed and analytical approach is given for simplifying the design procedure. By adding PIN diodes, frequency-reconfigurable BPF with slot structure, which is based on the proposed slot structure with multi-band bandpass frequency response, is also designed. For all these BPFs, fabrications and measurements are carried out and very good matchings are obtained which validating the effectiveness of proposed BPFs with slot structure. In the latter part, slotted antennas for wideband circular polarization is proposed and investigated. The method of adding circular patch loading or tuning stubs to eccentric annular ring slot (EARS) antenna for achieving circular polarization is proposed and examined. Parametric studies of these two kinds of CP antenna are also carried out. Based on the EARS antenna with tuning stubs, dual-band and polarization-reconfigurable CP antenna are designed by embedded tuning stubs or PIN diodes. For all these proposed CP antennas, II fabrications and measurements are carried out and very good results of frequency response as well as CP performances are obtained.