Publisher Summary When there are chromophores in photo-responsive or photo-sensory proteins, absorbing in the visible region, such as photoactive yellow protein (PYP), rhodopsin (Rh) for vision, which are not necessarily involved in the photo-biological reactions, the light absorption can lead to ultrafast and highly efficient reactions. Such photo-induced ultrafast reactions taking place in protein nano-space (PNS, space of a few nanometer scales involving the chromophore and surrounding protein environment interacting with it and probably facilitating its ultrafast and highly efficient photoreactions) are characteristic to those photoactive proteins. Elucidation of the roles played by PNS in those photoreactions is the most important urgent problem that confronts one during investigations. This chapter examines primary processes of photoreactions of PYP, which functions as a blue light photoreceptor for a negative phototaxis of the purple sulfur bacterium Ectothiorhodospira halophila, some FP’'s and Rh by means of the fs fluorescence up-conversion measurements. This chapter discusses latest results of fs fluorescence dynamics studies on PYP because PYP is very stable for repeated irradiation which induces photo-cycles so that the very accurate experimental results can be obtained rather easily and also the preparation of the site-directed mutants as well as the PYP analogues with modified chromophores are rather easy.