Flexibility and dynamics in polypeptides and proteins give rise to conformational and structural heterogeneity. Quantification of the level of heterogeneity is essential for providing deeper understanding of molecular mechanisms in protein folding. Fluorescence lifetime distribution generated by the maximum entropy method (MEM) provides an unbiased estimate of intramolecular distance distribution. This article discusses several examples where MEM-generated lifetime distribution brings out site-specific conformational and structural heterogeneity in proteins. Use of the information on intamolecular distance distribution in revealing molecular mechanisms of protein folding is also discussed.