Enormous funds are currently being invested in nanotechnology, yet very little is known about how its products and by-products can interfere with both end users and people involved in their manufacture. Similar scenarios are already widely known in the history of science, such as the exploitation of radioactivity or the controversial issue of genetically modified organisms. As nanoparticles are more or less voluntarily produced, they are almost uncontrollably disseminated in the environment and in organisms and thus constitute a growing concern. By describing the impact of nanoparticles (and microparticles) on human and animal health, this book offers the first criteria for preventing potential problems deriving from these particles.