Imaging single molecules in three dimensions

One original impetus for the development of single molecule microscopy was its application to cell biology. Recently, several groups succeeded in the first demonstration of single molecule sensitivity in living biological cells. Still, these pioneering studies were restricted in their applicability to minor parts of the cells, which could be considered as flat surfaces. Here, we will review strategies, which allow the determination of the full three dimensional position of single molecules. Such methods will be basic requirements for general single molecule investigations on living cells.