Functionalized gold nanoparticles (GNPs) with controlled geometrical, optical, and surface chemistry properties are the subject of intensive studies and applications in biology and medicine. The efficiency of biomedical applications of functionalized GNPs in therapy and diagnostics is determined by their intrinsic physiochemical parameters and by effective delivery of GNPs to target organs, cells, or subcellular components. This book discusses optical properties of GNPs, their applications in biological studies, analysis of data on the in vitro and in vivo biodistribution, immunological properties and toxicity of most popular GNPs. A multidisciplinary approach is incorporated throughout.