This article reviews our efforts in understanding dynamical fluctuations of both conformation and enzymatic reactivity in single biomolecules. The single-molecule approach is shown to be particularly powerful for studies of dispersed kinetics and dynamic disorder. New single-molecule observations have revealed conformational transitions occurring on a broad range of timescales, 100 mus-10 s, offering new clues for understanding energy landscape of proteins, as well as the structural and chemical dynamics therein.