Femtosecond Stimulated Raman Spectroscopy (FSRS) powerfully advances time resolved structural studies of chemical and biochemical reaction dynamics due to its unique time and energy resolution. By exploiting stimulated Raman probing as depicted in Figure 1, this technique is capable of recording high signal-to-noise Raman vibrational spectra with 20 fs time precision and 10 cm -1 energy resolution. Following a fs actinic pump pulse, a narrow bandwidth Raman pulse is used to produce a well defined virtual state energy in the probed molecular system. Then, a compressed 20 fs Raman probe pulse interrogates the sample, stimulating the Raman transitions which are recorded by dispersing the probe pulse onto a multichannel detector. The time resolution is set by the cross correlation between the two fs pulses which can be 20-30 fs or shorter and the energy resolution is determined by the decay of the Raman polarization. 1 Ground-breaking femtosecond stimulated Raman spectra 2 have been taken of the ultrafast and highly efficient isomerization in the visual pigment rhodopsin. The spectra elucidate the key role of hydrogen out-of-plane (HOOP) modes in the initial photoreaction, which rapidly lead the system to the ground state photoproduct, where the majority of the structural changes in the isomerization then occur. A comparison of the time-resolved HOOP frequencies with DFT calculations of various twisted structures reveal that while the 11=12 bond is formally isomerized in the initial ground state photoproduct, the retinal structure is still highly distorted. Thus, FSRS has revealed the intrinsic isomerization reaction coordinate of this ultrafast reaction, and elucidated the mechanism of the primary event in vision. FSRS has also been used to monitor electronic changes, such as intersystem crossing (ISC) between metal to ligand charge transfer (MLCT) states in the dye FIGURE 1. Conceptual schematic of the FSRS process indicating the temporal and energy relationships of the actinic pump pulse, the Raman pulse, and the Raman probe pulse. The temporal resolution is determined by the cross correlation between the actinic pump and the Raman probe and can be shorter than 20 fs. The energy resolution is determined by the decay of the vibrational free induction decay of the Raman polarization.