This thesis details the synthesis and characterisation of novel nanocomposite materials. The unifying theme of all the projects investigated, is the aim to combine metal or metal oxide nanoparticles with polymer systems. In order to investigate the structure of the materials produced, the extensive use of advanced electron microscopy techniques is essential throughout. Chapter 1: This introductory chapter outlines key themes that are relevant to all the areas of research in this thesis. Theory, background and applications are provided for the fields of nanoparticles, polymers, and supercritical fluids. Chapters 2, 3 and 4 each report a separate area of research. In each chapter additional theory and background is provided where appropriate, and previous literature is discussed. The aims, results and discussion of each research area are contained within the relevant chapter, as well as conclusions and future work. Chapter 2: Supercritical CO2 is used to impregnate optical polymer substrates with silver complexes, which are then decomposed to form nanoparticles. Chapter 3: Metal oxide nanoparticles are used to stabilise dispersion and suspension polymerisations, providing a method of recovering nanoparticles from aqueous solutions and embedding them on the surface of polymer powders. Chapter 4: Two alternative routes, for creating polymer microspheres surface decorated with silver nanoparticles, are compared. Chapter 5: A detailed description of the synthetic methods, equipment, and analysis techniques used in this research is provided. Chapter 6: A brief but overall conclusion to the research is given.