The formation of magnetic nanomaterials by biogenic processes is an untapped green chemistry approach to nanomaterial production. This chapter focuses on magnetite-based nanomaterials made through extracellular processes by dissimilatory Fe(III)- reducing and Fe(II)-oxidizing bacteria as these microbes have particular importance in global Fe cycling. Biogenic nano-magnetite is typically between 5 and 50 nm in size and as such exhibits different mechanical, electrical, magnetic and chemical behaviours compared to larger-scale equivalent material. Therefore, an environmentally benign process for the production of these materials, as offered by bacteria, is highly desirable. The formation of biogenic nano-magnetite is discussed, primarily focusing on Fe(III)-reducing bacteria as these are the most widely researched. The methods that can be used to tailor the magnetic nanoparticles to give enhanced reactivity involve using different mineral precursor phases, different ratios of biomass: precursor mineral and also the addition of different metals either within the magnetite structure or by arraying metals on the surface of the magnetite nanoparticles. The applications of these tailored magnetite nanoparticles are wide ranging and covered within this review are a variety of redox sensitive organic and inorganic contaminants that can be stabilized through a reductive transformation or degraded by biogenic magnetite. There are also biomedical applications for biogenic nano-magnetite. Nano-sized magnetite particles are produced by a wide range of Bacteria and Archaea and have potential uses in environmental remediation strategies and the challenges of process scale-up and the utilization of waste materials in the production of these nanomaterials are the next steps to harnessing this green technology.