The work embodied in this book started with the purpose of exploring the new and simple method of preparing metal nanoparticles of different shapes and sizes which were further applied in the field of catalysis and biological activities such as antifungal and thermal ablating agent. In addition to that in vitro toxicity studies were also carried out in order to access the biocompatibility of gold nanoparticles. The metal nanoparticles were prepared in aqueous medium as well as the aqueous core of the water in oil microemulsion was used as nanoreactor for the formation of nanoparticles.the size of the reverse miceller droplets of water in oil microemusion is in the nanometer range, so the particles formed in the aqueous core are also in the same scale. Moreover at higher concentration of surfactant the aqueous core of water in oil microemulsion changes jts shape from spherical to rod shape as a result of which the metal nanoparticles are also formed in rod shaped structure. Such type of metal nanoparticles can have potential applications in many frontiers including sensors, biosensors, electrochemistry, immunochemistry, drug delivery, enzyme therapy, gene therapy etc.