This chapter contains sections titled: Nanoparticles Particles and the environment Nanoparticles in soil Nanoparticles in water Nanoparticles in air Nanoparticles in medicine Nanotoxicology Nanotoxicology in humans and experimental animals Complications with nanotoxicological studies Molecular mechanisms of nanoparticle toxicity and cellular defense mechanisms Molecular mechanisms of nanoparticle-induced cytotoxicity Nano-epigenomcs and epigenetics Conclusion References