Three-dimensional (3D) imaging techniques to investigate the spatial composition of crystalline materials and biological specimens have over the last decades been steadily refined to their current nano-scale resolution. However, nearly all of the methods relying on X-ray or electron 3D tomography only deal with static samples and cannot convey simultaneous information about structural changes. One very promising approach in this direction is to make use of ultrabright state-of-the-art X-ray sources, so-called free-electron lasers (FELs), which routinely produce coherent X-ray radiation with unprecedented intensities. Combining the high spatial and ultrashort temporal resolution of X-ray pulses for a full 4D characterization of the complex samples has been a long-standing goal of material and biological science.Due to the ultrashort duration of FEL pulses currently no measuring technique is able to precisely determine their time structure. The first step of this project at the LCLS will demonstrate a technique capable of measuring the X-ray pulse duration for every single shot with a precision of a few 100 attoseconds utilizing photoelectron streaking spectroscopy. This method will in addition permit us to directly investigate the substructure of the FEL pulses on an attosecond time-scale.In the second step we want to apply these pulses for time-resolved X-ray absorption spectroscopy: A UV pulse initiates a molecular reaction and the X-ray pulse is used as a probe, in effect sampling the dynamics of the complex system in real-time.The project would firstly greatly profit from the knowledge on ultrafast optics the applicant acquired over the past 4 ½ years at the MPI of Quantum Optics and secondly provide him with first-hand experience in the novel field of FELs at the most renowned accelerator facility worldwide, strengthening the applicant's scientific profile substantially, especially in the context of the upcoming European X-Ray Free-Electron Laser (XFEL).