## 1 Introduction  
During the last decades, the development of X-ray light sources based on low emittance electron accelerators has enabled spectacular increases in the average and peak brilliances. Brilliance corresponds to the number of photons per phase space element of the emitted X-rays and is the parameter best describing the performance of these sources. Electron accelerators optimized for free-electron lasers (FEL) use low emittance injectors to create electron bunches, linear accelerators and electron beam optics to minimize the emittance growth during acceleration and transport, and bunch compression to generate ultrashort bunches. The resulting low emittance and high peak current of the electron bunches are the key performance parameters for these facilities. In undulator sections much longer than for synchrotron radiation sources, the electron bunches are transported with high precision and collimation to enable the self-amplified spontaneous emission (SASE) process leading to FEL gain and occurring in a single-pass of the electron bunch [1; 2]. In the SASE process, the electron bunch typically undergoes a degradation of its properties and cannot be reused for another FEL source. It is instead dumped at the end of the beam transport. FELs therefore, in general, are single-user machines making their operation costly and the access to them much more limited than storage ring sources where electron bunches are circulated and are reused by several insertion devices. Since X-ray FEL radiation is generated by an intrinsically coherent SASE process it provides huge pulse energies of 10 mJ and possibly beyond, and pulse durations as short as single femtoseconds. These properties correspond to peak brilliances eight to nine orders of magnitude higher than obtained by storage ring sources. Exploiting this brilliance in FEL experiments allows embarking on yet impossible X-ray experiments that will lead to interesting and valuable scientific and technological applications. Two technologies have been pursued to construct electron accelerators for FEL applications: warm, normal conducting machines and cold, super-conducting accelerators. The latter allowing acceleration of electron bunches at a much higher repetition rate, thereby boosting the average brilliance and enabling to distribute electron bunches to many FEL sources. The first short-wavelength FEL user facility starting user operation was FLASH at Deutsches-Elektronen-Synchrotron (DESY) in Hamburg (Germany), which uses super-conducting accelerator technology [3] and provides FEL radiation in the XUV and soft X-ray spectral region up to the water window [4]. In the following years, several normal conducting accelerator-based FELs started operation at SLAC National Accelerator Laboratory [5], ELETTRA [6], SPring-8 [7], and the Pohang Accelerator Laboratory (PAL) [8]. The SwissFEL facility at the Paul-Scherrer-Institute (PSI) [9] is nearing completion. The European XFEL [10; 11] employs the same super-conducting accelerator technology [12] used for FLASH and is currently under commissioning for first user experiments in 2017. European XFEL enables the acceleration of up to 27,000 electron bunches per second with an electron energy of up to 17.5 GeV (compare Table 1) which are distributed to several FEL sources. Starting from 2019 European XFEL will operate a regular user program with initially six instruments continuously receiving X-ray beams. At SLAC currently a 4 GeV super-conducting accelerator is under construction for LCLS-II, based on the same technology used for FLASH and European XFEL plus enabling continuous wave (cw) acceleration [13]. The article is organized as follows: Following a description of the overall European XFEL facility, we shall first describe the photon beam transport and photon diagnostics systems, before introducing the individual science instruments. Finally, an outlook to future developments of the facility is provided.