Nanometal-polymer composite films are hybrid materials with inorganic nanoparticles immobilized and uniformly dispersed into a polymer matrix. Nanoparticles such as gold and silver, used to enhance the optical properties of the polymers, are called `optically effective additives` as they lead to new functionalities of the polymer based materials. Au (Ag)-polymer and copolymer composite films are suitable for applications such as waveguides, color filters, thermo-chromic materials etc. The strong plasmon band of Au and Ag nanoparticles (Localized Surface Plasmon Resonance) in the visible spectrum, that originates from the excitation of plasmons by the incident light, makes noble metal-polymer nanocomposites particularly adequate for sensing and biosensing applications. Furthermore, association of Au and Ag nanoparticles of various shapes with poly(dimethylsiloxane) (PDMS), allows the use of nanocomposite materials for microfluidic biosensing as well. In this presentation, we will talk about the in-situ synthesis of Au-PDMS and Ag-PDMS nanocomposites both at the macroscale and inside the channel of a microfluidic chip. The in-situ synthesis of nanoparticles involves the reduction of gold ions in a gold precursor solution by the curing agent of the polymer. The kinetics of the reaction has been investigated and the effect of annealing temperature on the morphology of nanoparticles has been studied by using imaging and spectroscopic methods. The nanocomposite has been successfully used for sensing of antibody-antigen interactions, allowing the detection of various important proteins. Nanocomposies of metals integrated into other polymers such as PMMA, PVA, and PS have been synthesized as well by using UV, microwave and thermal reduction methods. The effect of the particle`s shape on the properties of nanocomposites and their sensing abilities has also been studied by synthesizing nanostar particles and integrating them into microchannels. Because of the biocompatibility and non-toxicity of gold and silver nanoparticles, their nanocomposites can be used for plasmonic detection of biological entities in cancer research.