Modern wireless communication Systems such as global positioning system (GPS) and Global System for Mobile Communications (GSM) are required to operate at two different frequencies apart from each other. Microstrip antennas can avoid the use of two different single band antennas by adding one or more resonant structures to the microstrip patch configuration. These additional elements may be in the form of slots or other patches. This thesis has introduced and investigated the design and simulation of a multiband microstrip antenna using different slots configurations, by studying the effect of loading different shapes of slots with different dimensions and numbers into a square MSA on the antenna multiband characteristics. The proposed MSA configurations are simulated and analyzed using Microwave Office Software (Version 3.22). Techniques for bandwidth enhancement are also studied and presented using a thick substrate with direct probe feed; different thicknesses ( h = 2, 3, 4, 5 and 5.95 mm ) are used, the maximum bandwidth produced by this technique using a thickness of 5.95 mm is 90 MHz (more than 6 % of the central frequency).The results of this work show that different square MSA designs with different patch dimensions and slots configurations operate on the dual frequency bands (L1 = 1.575 GHz , L2 = 1.227 GHz ) of the Global Positioning System ( GPS ) and the dual frequency bands ( 900 MHz , 1.9 GHz ) of the Global System for Mobile Communications ( GSM ). Multiband MSA patches operating on frequency bands of two different applications are presented in this thesis, these dual wideband MSA patchesoperate on the lower frequency bands [ L2 = 1.227 GHz , L5 = 1.176 GHz of GPS and 900 MHz of GSM ]; and the upper frequency bands [ L1 = 1.575 GHz of GPS and 1.9 GHz of GSM ] of the GPS and GSM applications. Microstrip antenna configurations with four slots loaded in the form of cut from the sides of the patch of the antenna introduced a multiband characteristic with triple frequency band performance and a total bandwidth more than 220 MHz, two of the three bands are centered in the same range ( L1 = 1.575 GHz , L2 = 1.227 GHz ) of the dual band GPS resonance frequencies.