Single molecule fluorescence spectroscopy is emerging as an extremely powerful and sensitive tool to study complex biological problems. Single molecule fluorescence measurements can extract useful information that is hidden in the ensemble averaged biophysical or biochemical studies by virtue of their wide range of spatial and temporal resolution capabilities. With these advantages, single molecule fluorescence spectroscopy enables us to monitor the conformational states and their dynamics in the form of statistical distribution or time trajectory of physical observables. This review illustrates how the single molecule fluorescence spectroscopy has been used to solve questions on the complexity and heterogeneity of protein folding and dynamics.