Gold nanoparticles have attracted great interests in the fields of biological and medical applications in past few years. Comparing with quantum dots and other materials, gold nanoparticles have been investigated and utilized in several biotechnology applications, such as sensory probes, drug delivery, and therapy techniques. In addition, gold nanoparticles can be excited by light at Near IR absorbing, which makes them to be the next generation contrast agents for diagnostic and phototherapeutic applications, such as two photon luminescence imaging, light scattering imaging, surface enhanced Raman scattering, and photothermal therapy. Moreover, by changing the particle size, surface chemistry, or aggregation state, the optical and electronic properties of gold nanoparticles can be adjustable and applicable for different uses, which present a promising potential in biological and clinical research.