X-ray free-electron lasers have recently broken the femtosecond barrier, which separates the motion of nuclei from the much faster motion of electrons in molecules and solids. Evolving from a cutting-edge R&D project to a new scientific program, attosecond XFELs are now producing time-resolved observations of coherent electronic phenomena with atomic site specificity and unprecedented temporal resolution. 

In my talk I will introduce the physics of X-ray free-electron lasers and the experimental FEL R&D program at SLAC. I will then present our results on attosecond pulse generation and its application to the observation of coherent electron dynamics in molecules. Finally, I will present our ongoing R&D efforts towards plasma-based attosecond sources, capable of combining the peak power of XFELs with the fractional bandwidth of state of the art few-cycle lasers.