Dear colleagues, This special issue of pss(a) comes with a series of 18 selected articles covering recent advances in the broad scientific field of “Engineering of Functional Interfaces”. Interfaces between e.g. different solids, between solids and liquids, or between liquids and gases are the places where we encounter a rich variety of physical phenomena such as current rectification, bilayer capacities, and surface tension. Although boundary effects often appear evident and we are familiar with many of them from everyday life, their in-depth understanding can be quite challenging and results also in new fundamental insights. A classic example would be Einstein's explanation of the photoelectric effect in 1905, which today is the basis of advanced surface-analytical techniques such as X-rayor UV-photoemission spectroscopy and all their refinements. Similarly, it was a long way to proceed from simple metal-to-metal contacts in electrical devices to uniquely engineered, buried interfaces in current microelectronics, and eventually towards molecular electronics. The latter field is still in the phase of fundamental research, but there is little doubt that these efforts will pay off in real applications in the future. Functional surfaces and interfaces play also a central role in the context of bio- and chemo-analytical sensors. For instance in gas sensors, surface-bound reactions result in changes of the electronic band structure and hence the conductivity of sensitive surfaces, usually made of selected metal-oxide semiconductors. When it comes to the detection of biological molecules such as proteins and hormones in body liquids or environmental samples, surface engineering is facing another challenge: These surfaces need to be functionalized with highly specific receptor molecules (such as antibodies or aptamers in case of protein recognition) while, at the same time, molecules that are not of interest need to be repelled from the sensitive area. Translating the selective binding into a quantitative, usually electronic sensor signal requires dedicated readout techniques, which are often derived directly from solid-state physics including e.g. surface plasmons, piezoelectric vibrations, or dielectric spectroscopy. A field with similarly stringent requirements regarding interface engineering is the development of medical implants ranging from passive elements such as drugeluting stents to active electronic devices including cochlear- or retinal implants. The surfaces need to be biocompatible in order not to provoke adverse body reactions while, vice versa, the implanted devices need to be reliably inert with respect to e.g. electrochemical corrosion effects. This holds especially for devices used for nerve stimulation by preferably capacitive currents. Hence, materials need to be chosen carefully and often dedicated coatings must be developed to suppress protein adsorption or encapsulation by fibrous tissues. Complementary to this, there is also a novel trend towards passive implants that can integrate easily with bones and tissues, aiming finally at implants that can be fully resorbed by the body. These were only a few examples out of many fields in which suitably engineered interfaces are ‘doing the trick’. Catalysis, energy conversion, corrosion prevention, tribology, and nanoscale heat transport are also topics in which typical interface phenomena play very prominent roles. Thus, we are dealing with a highly active and seminal field in which fundamental research and practical applications come to closest proximity. We are convinced that many more facets are waiting to be discovered: This makes research into task-specific interfaces an appealing topic for scientists from various disciplines, including not only physics but, for instance, also chemistry, materials science, and sensor development. With the yearly contribution on the “Engineering of Functional Interfaces” we would like to shed some light on these thoughts by offering up-to-date perspectives provided by many different research teams. The Guest Editors would like to thank all authors for their valuable contributions and we gratefully acknowledge the excellent support by physica status solidi and its Editorial Staff. We wish all an interesting and pleasurable reading. Guest Editors Theodor Doll, Hannover Medical School and VIANNA Research Center, Germany Torsten Wagner, Aachen University of Applied Sciences, Germany Patrick Wagner, Catholic University Leuven, Belgium Michael J. Schöning, Aachen University of Applied Sciences, Germany