The previous chapter focused on the quantum mechanical description of the interaction of light with matter with particular attention to electronic transitions. In this chapter, we will explore some of the consequences and utilities of electronic transitions in probing properties of molecules through various forms of spectroscopy and then show how these can be applied to study nanoscale phenomena. The emphasis will be on the concept of fluorescence spectroscopy since this has proven particularly useful in studies in the life sciences with applications in nanobiotechnology and bionanotechnology.