The ability of producing ultrashort X-ray pulses from X-ray Free-Electron Lasers is a very important asset in view of the study of ultrafast phenomena. The European XFEL is one of the major X-ray Free-Electron Laser facilities. It is driven by a superconducting accelerator, and it is therefore capable of delivering high-repetition-rate SASE pulses in its baseline mode of operation. In this contribution, we discuss several techniques for producing sub-femtosecond to attosecond pulses at X-ray Free-Electron Lasers, including laser-induced energy and density modulation techniques, with special emphasis on the European XFEL.