There is a very clever paper coming out in tomorrow’s issue of Science magazine using Raman  scattering (in which light not only changes direction but also its wavelength) to image the distribution of specific molecules such as a fatty acid, or a drug, within living cells. The beauty of it is that one can use the vibrational motion (which is quantized) of the target molecule, rather than having to add some marker or dye. With a pair of lasers and some clever pulse technology, one can detect the presence of the molecule of interest at a given time in a given location, and even quantitatively measure its concentration. And the method is sensitive enough to image physiological concentrations of metabolites in living cells.
Medical applications that Sunney Xie and coworkers have demonstrated include the monitoring of the uptake and distribution of omega-3 fatty acids in living cells, the imaging of skin and brain tissue sections, and the monitoring of drug delivery.
 Historical footnotes: Raman scattering was first described by Chandrashekhara Venkata Raman (1888-1970, Nobel prize 1930) 80 years ago, but it had to await the arrival of advanced laser technology before it could become really useful. Raman also studied the vibrations of larger units, such as musical instruments. The authors cite another historic paper that is even older than Raman’s, a 1917 publication on the phenomenon of stimulated emission, by a certain A. Einstein.
Reference: C. W. Freudiger et al., Science 2008, 322, 1857.