Proteins are natural machines that perform various functions that sustain our lives. To be biologically active, proteins, linear chains of amino acids, need to form compact three dimensional structures in the process called protein folding. The folded structures of proteins are determined by the primary sequence of amino acids. However, it is still extremely difficult to understand the relationships among the amino acid sequence, the folded structure, and the function of proteins. In our laboratory, we develop now single molecule fluorescence spectroscopy and observe the rapid process of protein folding directly. In addition, we observe the functional dynamics, a sliding motion along DNA, of a tumor suppressor p53. Furthermore, based on the knowledge of protein folding and function, we are developing a new strategy to design artificial proteins.