The rapid growth in second generation mobile telecommunications has resulted in the design of the third generation mobile telecommunication system together with proposals for the fourth generation system. Much of this has been driven by consumer requirements for a global telecommunications system, allowing connection from anywhere around the world and offering data rates that facilitate access to a range of multimedia services. Therefore, continual research into mobile handsets, improving standards and delivering smaller, lower cost devices, is required. The work presented reviews the different mobile technologies and highlights trends in design and application, presenting research into new configurations of RF front-end systems and antennas for mobile communications, specifically the third generation mobile telecommunication system. Developments in the integration of digital circuits has dramatically reduced handset sizes in recent years. However, the same progress has not been seen within RF circuitry. This work, therefore, investigates the properties of microstrip patch antennas for use within mobile handsets and the development of an integrated diplexer antenna. Having reviewed microstrip filters, further designs for a diplexer are also presented. The limitations of these designs are investigated and solutions proposed.