A critical need in the fi eld of nanotechnology is the development of a reliable and eco-friendly process for synthesis of metallic nanoparticles. A number of different organisms, including bacteria, microalgae, yeast and fungi, have shown their ability to produce metal nanoparticles. But they have some drawbacks in providing better control over size distribution, shape and crystallinity. This review article presents an overview of microorganisms (bacteria and microalga) capable of producing silver and gold nanoparticles. Also mechanisms of nanoparticles formation and parameters which infl uence their growth are discussed.