Intracellular trafficking represents one of the major limitations of current non-viral nucleic acid delivery approaches [155] . In other words a large percentage of intracellular cargo molecules are entrapped in vesicular compartments and thus will not trigger the desired effect. Moreover, degradation or retrograde transport might further reduce the number of active molecules. So in order to determine the overall efficacy of a given delivery approach it is essential to know the numbers of intact cargo molecules inside the cell along with the minimal numbers of molecules required to cause a particular effect. Based on such information one can easily calculate the percentage of bioactive molecules. In the following chapter we will briefly describe selected examples of variable suitability for a quantitative determination of nucleic acids in a cellular context.