Interfaces and surfaces play a key role in modern electronics and optoelectronics. The function of many devices is generated by the interfaces between conventional semiconductors, such as silicon or GaAs. The electronic properties of bulk semiconductors themselves are by far not as useful or spectacular. In contrast to standard semiconductors, many materials with complex electronic systems, such as superconductors, magnets or multiferroics already show in the bulk intriguing functional electronic properties. At interfaces involving such materials, the complex electronic interactions of the bulk can cause unique effects. The electron systems can be drastically altered; new electronic phases may occur, often with dramatic consequences for the electronic properties of the materials. In the presentation I will provide an overview of the startling phenomena taking place at the interfaces between complex electronic systems and discuss how fundamental questions bear a direct impact on applications.