Nanotechnology is the term given to those areas of science and engineering where the phenomena take place at nano-scale dimensions. Nanotechnology deals with particles sized between 1 to 100 nanometers in at least one dimension and it involves developing or modifying materials or devices within that size. Nanoparticles have different physical, chemical, electrical and optical properties than those that occur in bulk samples of the same material. All aspects of life will benefit from the revolution in nanotechnology. Engineered nanoparticles are increasingly produced for use in a wide range of industrial and consumer products. Hundreds of tons of nanoparticles already enter in the environment annually, but still very little is known of their interactions with biological systems. The challenge for toxicologists is to identify key factors that can be used to predict the toxicity, permit targeted screening, safe and sustained development and use of nanoparticles. In order to gain a sustained development, new technology always needs a good balance between benefit and risk. The aim of this paper is to summarize the target organ toxicity of nanoparticles in different biological systems.