The increasing performance of hardware devices, from CPUs to I/O peripherals, is moving the performance bottleneck towards the Operating System, which is in charge of multiplexing resources. As an example, modern InfiniBand adapters reach up to 100Gb/s, have a latency of few microseconds, which is comparable to the latency of a system call, and implement a large part of the network stack entirely in hardware. With such features, the adapter can perform most of the multiplexing and filtering actions the OS has done so far, and applications need very low software overhead to leverage full performance. Therefore, the OSes of the future require deep changes in their design guidelines. This talk covers the main challenges modern hardware poses to OSes, and reviews the main design guidelines proposed in the last decades as well as some recent efforts to rethink the role and the structure of the OS.