Nanoparticles show very high mechanical properties as well as many remarkable physical properties. Attempts to make new materials with excellent properties by adding these particles to polymeric materials, composites, ceramics, alloys, etc., have been mostly discouraged. During experimentation with nanoparticles some negative effects on human health were found. Insufficient studies of these effects created wide range of opinions, from “nanoparticles are worse than asbestos” to “nanoparticles don’t affect human health at all.” This chapter discusses some physical, chemical, and biological aspects related to nanoparticles and nanomaterials. It explains why the public has to be cautious in using nanoparticles until definitive investigation results are obtained related to humans and the environment.