## 1 Introduction  
X-ray free electron lasers (XFELs) represent a new generation of electron accelerators [1]. They produce bright bursts of X-rays at periodic intervals, where these pulses are both spatially coherent [2; 3; 4] and ultrashort in duration [5; 6; 7; 8; 9]. This combination of high brightness and ultrashort pulses produces extraordinary peak intensities, which has proven extremely attractive for certain fields of research in addition to creating entirely new fields which were not previously feasible, such as nonlinear X-ray signals [10; 11; 12; 13; 14; 15]. Due to the extensive experience amongst researchers with storage ring X-ray techniques [16], many of the first experiments [17] applied well-established methods, such as X-ray spectroscopy and scattering, but in a time-resolved manner, taking advantage of the ultrashort pulse durations to measure dynamics in matter [18; 19; 20; 21]. As experience with the facilities has increased in recent years, new techniques have been developed, including diffract-before-destruction methods, where the short X-ray pulse scatters from the sample before the atoms can move [22], providing the ability to measure room-temperature, radiation-damage-free structures. This ability has been applied to develop a technique called serial femtosecond crystallography (SFX), where a stream of tiny protein crystals is delivered into the focus of the XFEL, and, though the intense X-ray pulse destroys the crystal, its diffraction pattern is measured before the crystal is destroyed [23; 24]. These facilities are still in their infancy, with the first hard X-ray FEL only in operation since 2009 [25], but even in the few short years since their arrival they have attracted significant interest from researchers around the world, with the result that several new XFEL projects are underway worldwide. Here we present an overview of an XFEL project located in Switzerland, which is expected to welcome its first users in 2018.  
The SwissFEL XFEL facility is located at the Paul Scherrer Institute [26] (PSI) which is the Swiss national laboratory home for large-scale accelerator-based user facilities. It includes a 3 rd-generation synchrotron light source (SLS), the Swiss Muon Source (SuS), the Swiss Spallation Neutron Source (SINQ), and a high-intensity proton accelerator (HIPA). The SwissFEL construction was preceded by an intense R&D period, with the goal to allow for a very compact and economical design, and to have several features which are unique amongst the XFEL facilities presently in operation or under commissioning worldwide. In this review we highlight SwissFEL's expected capabilities and how it fits into the worldwide XFEL community. This article is organized into several sections: Section 2 covers the beam dynamics, injector, linear accelerator, undulator, and beam diagnostics components of the accelerator; Section 3 describes the X-ray optics, photon diagnostics, and experimental laser systems of the photon beamlines; Section 4 provides details on the experimental stations and their instruments, with a subsection on the 2D X-ray detectors available at the facility; finally Section 5 summarizes the infrastructure common to all aspects of the project, including timing and synchronization, motion control and data acquisition.