Nanoparticles are by definition too small to be visible in an optical microscope and devices such as scanning electron microscopes must be used to resolve them. However electron beams quickly lead to cell death and so it is difficult to study the interaction of nanoparticles with living cells in order to establish whether such interactions could be damaging to the cell. A simple modification to a conventional inverted optical microscope is proposed here which renders the location of nanoparticles readily apparent and permits tracking of them in three-dimensions. Particles in the range 100nm to 500nm have been tracked with a temporal resolution of 200ms. The technique, although motivated by the desire to study the interaction of nanoparticles with cells, has a wide range of potential applications in the fields of food processing, pharmaceuticals and nano-biotechnology.