Single-molecule spectroscopy is a powerful tool for studying complex systems with heterogeneity. With single-molecule techniques, properties of molecules can be obtained and analyzed individually and valuable information such as distributions of different states can be obtained with ease, unlike the conventional bulk measurement can only provide ensemble averaged value. Furthermore, the dynamical behaviors of molecules can be revealed without external synchronizations.  Owing to these advantages, single-molecule spectroscopy is widely used in the field of biophysics in pursuing the physical foundations of biological processes. Our lab uses single-molecule fluorescence resonance energy transfer (smFRET) microscopy and magnetic tweezers (in construction) as major tools to tackle the complicated biological processes.