Fluctuations are the hallmark of biological systems in action. Compared to man-made machineries, which work in deterministic manner, biological systems fluctuate at various levels, from an individual molecule to the whole cell as a system. These fluctuations are a result of the small energy scale of the biomolecular contacts like the base-pairing of nucleic acids or the folding of proteins, which is comparable to the thermal energy. They are not merely imperfections in the biological machineries but often key elements in their working mechanisms, as the living organisms have evolved to optimally utilize the fluctuations. Single molecule approach directly addresses the real time dynamics of a separated molecule or a complex of molecules and thus provides direct access to their fluctuations under physiological conditions. I will demonstrate the critical role of fluctuations at the molecular level in two examples of biological systems: the transcription initiation by the RNA polymerase and the assembly of the ribosome. Understanding how biology utilizes the fluctuations will lead to further insights into the design, function, and regulation of the biomolecular systems, with distinctive features from those of artificial machineries.