Free radicals are unstable atoms or molecules with free outer electrons. This makes them highly reactive because free electrons always strive to form a stable bond. This stabilization involves gaining an electron from another molecule, triggering a chain reaction. Such reactions are omnipresent in the human body, but under certain circumstances can damage biomolecules. Whether nanoparticles are intracellular taken up leading to the activation of free radical production is currently being discussed. Ongoing studies are investigating whether the amount of free radicals formed on the surface of nanoparticles is sufficient to induce cellular effects. This dossier provides an overview about what free radicals are, how they originate, why organisms need them, how they are neutralized, and what we know about the connection between nanoparticles and free radical production.