After twenty-five years of development, single-molecule techniques have become popular tools to study biology. There are two broad families of single-molecule approaches: fluorescence-based detection and force-based manipulation. I will present two systems to demonstrate the power of these approaches in dissecting the molecular mechanism of complex biological processes. First, optical trapping experiments showed that a bacteriophage genome packaging motor simultaneously rotates and translocates the DNA genome during the viral life cycle. This homomeric ring ATPase displays a remarkable level of coordination and an unexpected division of labor among its subunits. As DNA is packed into the viral capsid and the internal pressure rises, the motor gradually changes its step size and the amount of DNA rotation in order to achieve robust and efficient packaging. Second, single-molecule FRET assays revealed that the HIV reverse transcriptase is able to flip and slide on nucleic acid substrates. These large-scale motions allow the enzyme to quickly switch between various binding modes and perform different functions.